Our GMC 3500HD has a GPS and so do our phones, but we can't rely on either of them completely. There are a number of challenges with GPS's. The first being connectivity.
Phone-based GPS systems like Google or Apple Maps are useless if you are Boondocking or anywhere that does not have data connectivity. I have had many occasion to be out somewhere and the phone goes haywire and has no idea where it is. Most of the time that is due to being outside of cellular coverage, but any interference with reception such as driving through mountains or the city will cause issues with accuracy. Internet connected apps work so well it is easy to forget that the map data is in the cloud. When cloud access is lost the maps are lost as well. Turn your phone into Airplane Mode to see exactly what happens when data connectivity is lost.
There are apps that have databases that download and install onto the phone. CoPilot GPS is one that is used fairly often in the traveling camping community. The challenge with CoPilot and many of the apps is they license their routing algorithms from third-party providers. Licensing is expensive. At the end of the day someone is collecting the height and width of every road to make routing work. Small businesses like CoPilot do not generate a lot of revenue. The end result is they use suboptimal routing systems that can get you into a situation you may not be able to get out of. Apps are nice for planning, but not for actual driving.
Our truck GPS is fantastic. Many of the issues such as data loss and routing work very well. GM can afford to license legit technology that works for most of their customers. The problem with the vehicle GPS is bridges, tunnels, tight roads, etc. Onboard GPS systems simply do not take into account anything other than the vehicle it is installed in when routing. It is another great system for planning, but it is useless when actually on the highway.
So what do we use? The industry standard system – Garmin. Garmin is a Goliath company. They produce GPS systems for Aviation and Transportation. The systems they build are rock solid and reliable. They license their data from a company called Here Technologies that maps planet earth. Here generates millimeter accurate data for locations throughout the world. Their coverage far exceeds Google and anyone else that maps roads. Garmin is a smart company. They license data for everything from Here, including bridge heights, traffic, road grades, railroad track locations, etc. Here is a Big Data Internet of Things company. They map everything. When you are using a Garmin unit, you have access to the same data points.
The system we actually use is the Garmin RV 785 with Traffic. The system not only takes into account the length, height, width, propane, etc. it also has an onboard dash cam that saves money from having to buy a camera to record the road in front of us. We highly recommend Garmin products. The best of the best for RV travel is the 785 with Traffic. Here's the link to Garmin 785 system we use.