How to Drain Gray and Black Wastewater Tanks

RV waste management is the most dreaded task a camper owner must deal with. No one likes to talk about sewer lines and waste disposal, but it is one of the most important systems in a coach. If holding tanks are not properly managed it can cost thousands of dollars to repair and take a camper completely out of service for weeks if not months.

If you are reading this I'm sure you know that your RV has holding tanks, but you may be one of the countless people that were not properly trained on how the systems work when you bought your rig. Don't feel bad if you are. Every new RV owner has faced this challenge. I certainly had no clue when I bought my first rig. I had to ease drop on a few of my campground neighbors and experiment to figure it out.

After reviewing this article, you should be able to manage your sewer and waste water systems like a pro.

Gray & Black Tank Video Guide

Complete Guide on how to Manage and Flush RV Gray and Black Water Tanks

How Does the Sewer and Waste Systems Work in a RV?

We would never send our teenage kid onto the road without knowing how to read a fuel gage and fill up with gas, yet most RV buyers have never been properly trained on how the sewage and water waste systems work. We buy our rigs and are sent down the highway. The vast majority of new RV owners find themselves at a campground or dump station not knowing how to connect or operate the sewage disposal systems.

Let's try to clear some of this up.

Holding tanks

There are some RVs that have composting toilets, but these are generally installed aftermarket. For this article I will focus on the average camper that has standard wastewater systems.

Wastewater Holding tanks are separated into two categories – Blackwater and Graywater. Blackwater tanks hold waste from the toilets and Graywater tanks hold water from showers, sinks, dishwashers, washers, etc.

In some small RVs it is possible to only have one tank where all water including the toilet is drained. Any waste water tank that holds toilet water is always defined as a Blackwater tank.

RVs with water systems also have Freshwater tanks. These tanks hold clean water used in sinks, showers, toilets and anywhere that requires fresh water.

The Freshwater tank feeds clean water to all areas in a coach. The runoff water is then directed into one of the two types of holding tanks. For example, when a sink faucet is turned on clean water will be siphoned out of the Freshwater tank and then drain into the Graywater tank. Water used to flush a toilet is also pulled from the Freshwater tank; however, when the toilet is flushed the wastewater is directed into the Blackwater tank.

In most RVs, the Graywater and Blackwater tanks are often the same size and work the exact same way. The primary difference between Black and Gray tanks is what waste is directed into them and where they are mounted. Black tanks generally sit directly below or very close to the toilets. Gray water tanks tend to be mounted in various places under the floor of a coach.

Blackwater tanks must be mounted below and close to a toilet because solids are dropped into them. The waste tube would clog up quickly if the contents from a toilet had to navigate through twists and turns.

Graywater tanks can be mounted just about anywhere as long as they are below the lowest drain (e.g. shower). PVC plumbing pipes like we have in a house are used to direct dirty water into the tanks. Gray water plumbing lines can make bends and turns so the tanks do not need to be directly below sinks and bathtubs. Solid materials such as food particles from dishes should never be flushed down a sink drain.

Graywater Tank Management

Graywater tanks receive more runoff water than Blackwater tanks. They generally fill up fast. Taking showers and cleaning dishes are the two activities that generate most of the gray waste water that flows into a Graywater tank. When the Gray tanks are full, they have to be dumped or runoff water will backflow into the camper through low drains such as the shower.

When dry camping it is important to limit water consumption as much as possible

When one is staying at a full-hookup campsite, the gray water tanks can be left open and draining into the parks sewer system. When connected to a RV park, Graywater tanks do not typically need to hold water. The waste gray water is drained out of the coach like a house.

Blackwater Tank Management

Blackwater tanks generally only hold waste from the toilets. Blackwater tanks are generally the same size as the Graywater tanks, but they can take quite some time to fill up. Most people can camp for two weeks or more without filling up a Blackwater tank.

Gray water tanks and fresh water tanks are generally what limits off-grid camping. Black tanks can last for weeks before filling up.

Blackwater tanks need ample water in the tank to help breakdown toilet paper and solid waste into smaller particles that can be evacuated out of the sewer line.

The #1 issue with a Blackwater tank is that the water level is kept too low and solids cannot break down

Flushing a toilet is fast in a camper. RV toilets are actually more closely related to how an outhouse works than what we find in a home. Waste from a toilet drops directly down into the Blackwater tank.

RV Toilets do not flush like one in a home. The user decides how much water to use per flush. It is common to see people step on the drain lever and release it quickly without letting sufficient water run into the tank.

Low water levels in a Blackwater Holding tank will eventually create a condition commonly referred to as a “Poop Pyramid.”

Poop Pyramid is a funny sounding name, but the damage that can be caused by piled up solid waste in a Blackwater tank is very difficult and expensive to repair.

What is happening in the “Poop Pyramid” situation is solids drop directly below the toilet into the Blackwater tank. If the waste is not completely immersed in water it will start to dry out and bond to the bottom of the tank. As more solids are flushed a triangle shape is formed that looks like a Pyramid.

RV Poop Pyramid

The diagram above illustrates what is happening. The solid waste exiting the toilet creates a pile that extends above the top of the water level. The solids dry out creating a hard brick-like material. If left unchecked, the solids will develop into a virtually indestructible waterproof structure. As the Pyramid grows in size it will eventually clog the bottom of waste line exit hole at the top of the tank. At this point the toilet will back up and not function.

Plungers like we use in a home will not work on an RV toilet!

Suffice of to say, we do not want Poop Pyramids forming in our Blackwater tanks. The easiest way to prevent this from happening is to add several gallons of water into a holding tank after dumping it and then use ample water when flushing to ensure that the water level is always above the waste.

Water will break down toilet paper and solids into particles that can can be drained when the tank is dumped. If solids raise above the water line they will dry out and build a Poop Pyramid.

Campground Sewer Line Connections

Most RVs have a single dump line that is usually located somewhere in the middle or back corner of the rig. The waste lines are always on the Left side of the coach, which is referred to as the service side.

RV Sewage Drain Line

In Class-type Motorhomes and Buses the waste line may be installed in a compartment that is on the left side of the coach typically forward of the rear tires.

Class A Sewage Drain Line

Although the waste lines may look somewhat different, they all function basically the same way. RVs typically have quarter turn twist on/off fittings called lugs that are designed to make it easy to connect and disconnect hose extensions that stretch from the coach to the dump station.

Most rigs have some sort-of cap that covers the connection point to the rig. These caps have rubber seals that can stick and make them difficult to remove. Twist the cap LEFT to remove it and RIGHT to install.

Camco makes a nice Sewer Fitting Wrench Set that is good to have. The tool locks into the connectors creating leverage that makes it easier to remove caps and fittings. NOTE: in position pictured below he has the tool set to loosen or remove the hose from the fitting.

It is very important to keep in mind that the caps are there to prevent water and waste from dripping out while traveling. DO NOT stand in front of the cap when removing it or you may end up with waste water pouring into your lap or onto your shoes!

It is a very good idea to wear high quality Gloves any time you are working around sewage systems. I personally use this set of Heavy Duty PVC Work Gloves. They are completely waterproof and designed for people who work with harsh chemicals. The gloves have a solid textured finish that makes it easy to grab onto fittings and hoses without them slipping out of my hands.

Sewer Line Connections

The first step in setting up your coach to drain its tanks is to connect the RV to the dump station drain hole. The connections are generally standard on the RV end, but they are not always the same at a campground or dump station. The dump station could also be several feet from the RVs sewer line connection as well.

Hose Extensions & Fittings

Some RVs come with waste lines, but they are almost always garbage. When dealing with sewage lines we don't want our hoses to disconnect or leak. We also need to ensure we have long-enough hoses to reach most dump stations. At a minimum, one should carry at least 20 feet of hose. That will reach most dump stations, but not all of them. I personally carry 30 feet of hose to be safe.

I'm not trying to be a Camco commercial, but I have found that they make the best standard sewer-line fittings for campers. They sell practically every possible combination of connectors that accommodate nearly every situation. I like sticking with the same brand when dealing with waste water so I can ensure that everything will fit and seal together properly.

Camco sells two different types of waste line hoses. I personally prefer their RhinoEXTREME hose extensions. The hoses are made with high quality crushproof parts that hold up better than other hoses I have tried.

The Camco RhinoEXTREME kit pictured above comes with a Bayonet Elbow connector that will fit almost all dump stations. The connector will twist and lock onto most stations making a secure leakproof seal, but it can also be pressed into a dump station that has stripped or missing threads. Most of the time this kit is all one needs to dump their rigs.

I personally bought two of the RhinoEXTREME Hose Kits so I would have 2 15 foot hoses that can be connected together to make 30 feet if needed. This also gave me two Bayonet Elbow fittings that is nice to have in case one breaks or is lost.

Another nice component to have on hand when connecting to various dump stations is a Camco Sewer Hose Seal. These rubber donut shaped gaskets slide onto the end of the RhinoFlex Bayonet Elbow that comes with the kit above to create an air-tight leakproof seal. These are not always needed, but they are cheap and nice to have on hand.

The final product I like to use when draining RV tanks is the Camco 45 degree clear elbow.

I believe a clear 45 degree elbow fitting is essential when connecting to a rig. One end of the elbow connects directly to the exit line coming from the camper. The hose then connects onto the other end of the 45 degree elbow. This device provides three key benefits:

  • You can see what is going on in the line. Without a clear sightline it is possible that water could be streaming into the line without knowing. The clear elbow lets you see exactly what is going on between the camper and the hose.
  • The rigid hard plastic catches and redirects the water pressure exiting the RV. The direction change helps slow the water down before entering the hose, which helps prevent leaks and hose blow-outs.
  • The 45 degree angle of the elbow helps improve waste line life. Without a 45 degree connector, a sewage hose has to bend nearly 90 degrees when connected to a camper twice (once at the camper and once on the ground). The hard angles put stress on the hose that will eventually cause it to fail at the worst possible time — when it is under pressure with a high volume of water flowing.

Sewer Connection Procedure

Connecting the hoses and fittings together is a simple lego-like process. Every hose and fitting has a male and a female end. The female lug connectors on one end twist onto the male lug connectors on the component it is connecting too.

RV Lug Male/Female Comparison

It generally only takes a few minutes to connect all of the lines and fittings together. It is important to fully twist the connections together to ensure a secure leak-proof connection.

RV Sewer Hose Connected to Camper & Dump Station

The picture above isn't exactly the same parts we have been discussing, but it shows how everything connects together. The components fit into each other creating a relatively leakproof seal. When a RV is connected to a dump station it is very similar to how a home is connected to a septic system. The lines and fittings are there to ensure that the waste water is routed out of the RV and into the dump station efficiently.

It is important to use high quality fittings and lines so you don't have leaks. High pressure water flowing from a camper can rip low-quality hoses in seconds making a big mess. Trust me on this one — buy the best hoses and connections you can find!

Lug Fittings and Challenges

The lug-type fittings we have been discussing are the RV industry standard connection type, but they are not the standard for high-pressure hoses in any other industry. Fire trucks, Port-A-Potty, swimming pools, etc. use compression fittings called Cam-Locks. Cam-Locks cannot be accidentally disconnected and it is nearly impossible to get one to leak.

Lippert (LCI) is a company that is huge in the RV industry. LCI acquired a company in 2014 called Waste Master that made a kit to convert problematic RV Lugs Fittings to reliable truly leakproof compression Cam-Lock fittings. Lippert put their engineering and production talents to work making the LCI Waste Master System readily available to consumers.

I made the transition to the LCI Waste Master System in our coach. This video walks through the procedure to change to the Waste Master Cam-Lock hoses and fittings. This change does require some plumbing skills, but it is something that can be done by any handyman or RV tech. I installed our system at a campground.

Draining Holding Tanks

Now that we have our hoses connected to the sewer, it is time to review the procedure on how to drain the Gray and Black tanks. The Gray tanks are generally less problematic than a Black take so we can start there.

Gray Tank Dumping Procedure

Gray tanks are designed to catch run-off water from sinks and showers. The water is soapy and dirty, but it should not have any solid materials. A properly maintained Gray tank is very quick and easy to dump.

Food particles and other debris should not be drained into the Gray tank. It can damage sensors, create unpleasant odors and make the difficult to clean. To ensure we don't make the mistake of sending unwanted stuff into our Gray tanks, we use strainers in our showers and sinks.

RV's have different sizes and configurations of Graywater tanks. Most only have one, but larger RVs may have two or more tanks. Campers generally have a service bay where water connections and drain knobs are located, but it is very possible that not all drain valves are located in the compartment.

RV Manufacturers distribute holding tanks in the frame to help manage weight. Tanks that are mounted further away from the service compartment may have their dump valves in a different location or even under the coach. It is very important to look under your rig and in all compartments on the left (service side) of your coach to ensure that you have identified all drain valves.

Grand Design Nautilus Water Management System

The above picture is of our Grand Design Momentum 397TH Nautilus Service bay. All RV manufacturers set their service bay up differently, but the components generally do the same thing (fill water, winterize, sanitize, drain, etc). In our coach, there are three handles at the bottom of the service bay that are labeled Black Tank (Left behind water filter) and two that are Labeled Gray Tank.

There is not an official standard on organizing service bays, but generally when you see two handles side by side they should be in the order from Left to Right as Tank 1 and then Tank 2. Some RV Manufacturers mount their drain lines vertically vs horizontal like pictured above. When the handles are in a vertical orientation, the top handle is usually Tank 1 and the one below it is Tank 2. It is rare to find more than two Gray or Black tanks in a camper, but it is possible.

The drain handles in the above picture are in the closed position. Pushed in is closed and pulled out is open.

Time to pull the handle!

The fun part of draining the tank starts when we release the water. We just need to be very careful when doing this. Water is heavy and powerful when it starts to drain. Opening a handle quickly can create more pressure than the hose and/or fittings can sustain. We need to open the pull valve about 1/4 of the way first and then monitor what is going on through the clear fittings we installed earlier.

Both Gray tanks can be drained at the same time, but it is best to drain one tank at a time so you can monitor the process and ensure all is working correctly. If you encounter an issue, it is easier to determine what tank has a problem if you drain them in sequence. It doesn't matter if you start with Gray Tank one or Tank two.

The water will start to flow slow when cracking open the drain valves, which can be misleading. The PVC Pipes from the tank to the exit point can be a significant distance. We need to give the water time to develop a full and even flow before opening the valves further. When the water is flowing at a consistent rate and you have verified that the hoses and fittings are good, it is safe to open the valve in increments (e.g. 1/2, 3/4, full).

Campground Gray Tank Management

Gray tanks fill up fast — especially when connected to a virtually unlimited water supply at a campground. If you are camping in a warm location (above freezing day and night), you can leave the gray tank valves open and let the gray waste water continuously flow out of the camper into the campground sewer system.

When the valves are left open, the coach functions very similar to a house. You do not need to monitor tank levels or worry about how much water is being consumed. This is the main reason why RVers like full hookup sites. It takes the stress out of worrying about running out of water or filling up the tanks.

The Campground Exception

There are two conditions where you do not want to leave the Gray tanks open at a campground or full service hookup site.

  1. When the temperature is close to or below freezing the valves should be closed — unless you are emptying the tanks. RVs that are designed for cold weather have their tanks mounted in areas that should not get below freezing as long at the heat is on inside of the coach. Cold weather RVs also generally have heating pads mounted on the holding tanks that keep the water warm so it will not freeze.

The lines from the tanks to the drain penetrate the bottom of the coach and are directly exposed to cold air. Water in the drain line will freeze and can damage the PVC pipes, fittings, and connectors. The cold air can also make its way up the line and super cool the water release valve making it impossible to open or close.

When camping in cold climates, it is a good idea to disconnect the drain lines and close all connections when you are not draining. Store everything inside, let the water tanks fill to 2/3's or more in the tanks and then drain. Rapidly flowing water when draining will not freeze — stationary water left in lines exposed to freezing temperatures will.

  1. When your Black Tank is 2/3's full or needing to be drained. Gray water pressure is needed to fully clean out the PVC drain pipes after emptying a Blackwater tank. When nearing the time to dump a Black tank one should close the Gray water valves and allow water to accumulate.

The general rule of thumb is to let the Blackwater tank fill up to 2/3's full before emptying it. The water in the tank helps break down the solids making it easier to drain. It can take a day or two to fill up a Gray tank, so closing the valve when the Black tank is at 2/3's should provide sufficient time for the Gray tank to fill without being at risk of the Black Tank overflowing.

Black Tank Management

Dumping the Blackwater tanks is similar to dumping the Graywater tanks, but Blackwater tanks have some quarks that we need to be aware of. We cannot dump a Black tank and go like we can with Gray tanks. We need to ensure that a Black tank is fully dumped, clean and prepped so we don't develop a “Poop Pyramid” like we discussed above.

Keep Ample Water in the Blackwater Tanks!

I have mentioned this throughout this article, but it is worth saying again. Solids that are flushed into a Blackwater tank need to be fully submerged in water at all times. Toilet Paper and Poop that is above the water level will start drying out and form a solid material that cannot easily be drained from the tank.

Blackwater tanks are ventilated. Air is designed to circulate through them and vent through the roof to reduce smells in the rig. The constant airflow accelerates drying out everything that is above the water line. The air circulation also evaporates some water that is held in a tank over time. We need to use black tanks regularly to ensure ample water is in the tank. RVs that are going to be stored for extended periods of time must have their Black tanks fully dumped and cleaned.

Black Tank Dumping Procuedure

To properly dump a Blackwater tank, we need to fill up our Graywater tank. Graywater is used as the last step to clean the plumbing lines. We need to have as much Graywater in the tanks as possible so we can get good pressure to push out any debris left behind after dumping a Black tank.

The general rule of thumb is to start planning for a Black tank flush when the tank is around 2/3's full. 2/3's ensures that there is ample water in the tank and it usually takes quite some time to get to that level. The longer solids are submerged in water the more likely they will be broken down and suspended in the water vs laying on the bottom of the tank. It is also very helpful to plan on dumping a Black tank after a road trip. The sloshing around of the water inside of the tank helps break down the solids as well.

Black tanks take quite awhile to properly dump, clean and prep. It is best to plan dumping when you are at a full hookup site in a campground. There are often lines of people waiting at dump stations and not all of them have clean water that may be needed to flush and prep the tank.

Time to Pull the Handle!

At this point we have the sewer lines connected, the Black tank is 2/3's or more full, we have inspected all fittings and we have ample Gray water stored to help clean the lines when we are finished.

The next step is to open the Black tank valve, but the procedure is different than what we discussed in the Gray Tank section above. Black tanks need to have as much pressure as possible flowing out to help clean the tank. The gunk that is held in suspension in the water also creates friction that slows down the rate of flow.

When dumping a Black tank, we need to fully open the valve in one clean motion. The valve should be opened quickly, but not too hard. We don't want to break a handle, cable or valve. The idea is to pull it so it glides open with a smooth and even pull until the fully open stop is felt.

A bit of Graywater leaking is not the end of the world, but we most certainly do not want to spill the contents of our Blackwater tank on the ground. It is Very, Very important that you are monitoring the hose and connections while you are opening a Black Tank dump valve (don't look at the valve, look at the exit point and connections). If you see anything that looks out of place close the valve immediately and inspect.

It is nice to have a second person that can keep an eye on the hose and connection as well, but realistically most of the time we are going to have to do this on our own. Approach draining Black tanks with extreme caution. If anything looks out of place close the valve immediately and inspect. Stopping the flow is not instant. The shutoff valve can be several feet from the exit point.

Flush the Black Tank

Although we have drained the black tank, the tank is not actually clean. Solids and other debris end up sticking to the tank and they do not get flushed out. Toilet paper can stick to sensor inside of the tank causing them to stop reading properly. Solids will start drying out and create a near-permanent bond to the sides and bottom of the tank.

We need to flush the black tank with fresh water to help clean it out. Most RV's the a Black Tank Flush fitting that makes this process easier.

Black Tank Flush Inlet

The Black Tank flush fitting connects to a fresh water hose that channels clean water directly into the Black Tank. The pressurized water sprays against all sides of the tank helping loosen the debris and submerge them into water making them easier to drain.

One of the challenges of flushing is that it is very difficult to capture the debris and encourage it to flow out of the drain opening. The contaminants need to be fully suspended in water to make it easier to evacuate out of the tank. To properly do this, we need to close the black water tank valve and allow the water flowing in through the Flush valve to fill up the tank. When the water fills the tank it covers more surface area. The water level increase also creates more pressure that will help push contaminants out of the drain when the release valve is opened.

The steps to flush the tank are:

  1. Connect a water hose to the Flush Valve (do not use a drinking water hose)
  2. Close the Black Tank release valve
  3. Turn on the water and allow it to flow into the tank
  4. Monitor the tank level at the control panel inside of the RV
  5. When the Tank monitor indicates that the tank is 2/3's full turn off the water
  6. Open the Black Tank release valve and visibly inspect the color and cleanliness of the water coming out of the tank
  7. If the majority of the water flowing out of the tank looks dirty, let the tank fully drain and repeat the above steps

The water coming out of the black tank after flushing will never be perfectly clean, but after 2-3 full flushes it should be primarily clear. An excessively dirty tank could take 4 or more flushes. Keep flushing until you do not see any solid particles flowing out of the drain.

Parts I use for Tank Flushing

I thought you may find it helpful to see some of the parts I use for flushing our tanks.

Ready Black Tank for use

Now that we have the Black Tank clean, the final step in the process is to ready it for use.

  1. Verify the Black Tank Dump Handle is in the closed position.
  2. Add 2 Gallons of fresh water through the toilet
  3. Add 1 Gallon of Water to the toilet tank (fill to middle ring)
  4. Sprinkle 1 scoop of Happy Camper into the toilet water and let dissolve
  5. Aggressively flush toilet releasing the 1 gallon of Happy Camper treated water into the Black tank
  6. Add 1 more Gallon of Water into the toilet tank and then sprinkle 1 more scoop of Happy Camper into the water
  7. Flush the tank releasing the second batch of treated water into the Black Tank

The key to prepping a Blackwater Tank is to ensure that it has ample water to cover solids deposited into it. The 2 gallons of Fresh Water plus the 2 gallons of Happy Camper treated water should be sufficient; however, if you are going to be staying for extended periods of time in the same location or expect to have heavy use of the toilets add 1 or two more treated batches of Happy Camper to the tank.

When Boondocking we all want to conserve water, but the toilet is not the place for conservation. We need to keep the water level above any solids in the tank. Use ample water when flushing solids into the tank.

Final Thoughts

With proper care and maintenance the Gray and Black tanks should last for decades in an RV. The best way to insure that all of the steps are followed every time the tanks are filled and flushed is to create a checklist. It is also important to let visitors to your camper know how to properly flush the toilets. If you are uncomfortable having these types of conversations with guests, post a flushing procedure placard on the wall by the toilet.

A good way to verify that enough water is being flushed is to teach people a counting system. For liquids only (e.g. #1), have them hold down the flush bar for a count of 1 to 2 (i.e. count off — One-one-thousand, two-one thousand). We want to add about 1/4 to 1/2 gallon to the tank. For people depositing solids (e.g. #2), flush bar for a count of 3-4 while brushing the toilet bowl with a brush. We need to add 3/4 to 1 gallon of water into the Black tank after depositing solids.

Jim Kerr

Jim Kerr is a entrepreneur that has founded several businesses including Orbitz, Team Convergence, Assure Flight, and Passion Highway. He is an airplane pilot, PADI SCUBA Dive Master and adventure traveler. Along with his wife Lisa, they travel North America in their 2020 Grand Design Momentum 397TH Toy Hauler with their cat Dexter. To find out more about Jim, visit

Recent Posts