We use a combination of two different devices. The first one is a physical device that screws onto the tank valves. It has a pressure meter that reads the approximate state of charge. It also protects us from a rapid propane leak that could happen if a line comes lose or is cut. It is called a GasStop Emergency Shutoff Valve. The GasStop is a must have device. We are carrying 60 lbs across two tanks of Propane. A leak could create an explosion.
The second and more convenient device to use is a wireless system called Tank Check Dual. The system is genius. It has two wireless transmitters that magnetically stick to the bottom of each propane tank. The sensors send a signal into the tank and it measures the level of the liquid propane in the tank. The measurements are then sent via a Bluetooth signal back to a battery operated receiver that can be mounted just about anywhere. The level meter alone makes the system worth every penny, but what is cool is they also have an App that lets you read the level of each tank from a smartphone. It is very nice to know how much propane you are using at any given time and if you are going to make it through the night on a tank.