Don't know how this works for other RVs, but for Jayco & Starcraft, it is a well known problem. They have overflow tubes that extend below the tank and can easily lose 1/2 the volume of water on the way to a campground, and you don't even need to be on a bumpy road. Problem is solved by putting a valve on the overflow tubes. Pressure is not a problem because they also have a vent tube located up high adjacent to the fill tube. You just have to remember to keep the valve on the overflow tube open when filling. Once you get full and water starts coming out the overflow, close the tube. Don't have to open it again until next time you fill. Problem solved.
