5th Wheel Hitch, Aux Fuel, and Air Bag Questions

The truck's computer is probably confused as to why the stock tank keeps staying full when the truck has been using fuel. Maybe there is a programming fix for the gauge, available from the aux tank manufacturer?

Or maybe you will need to eliminate the gravity feed valve and use a pump and filler nozzle system instead, so the stock tank only fills when you fill it, like filling at a gas station, instead of staying full all the time.