2001 Beaver Patriot Thunder fresh water tank overflow

Is the pipe with the cap on it at the rear of the tank maybe about 1/2 in in dia and about 1-1 1/2 ins long? If so I would remove the cap and install an overflow tube the same dia as the pipe, rout the tube with an arch a little higher than the tank. When you fill the tank the water should pour out of that tube(not trickle) to prevent the tank from expanding and breaking the mounting tabs. Sounds like someone tried to cap the outlet to prevent water loss when traveling.