This might be a helpfull bit of information. Today I tried to add water to a camper I purchased recently. There was a lot of burping at the fill port, and I finally gave up after only getting a small amount of water in the tank.
I disconnected the overflow tube from the water tank and ran a coat hanger down through it. It had been plugged up with who knows what. After clearing and re-attaching the overflow tube, I was able to add water to the fresh water tank without further difficulty.