Second problem camping this weekend

[Trailmanor George]

The second problem we had this weekend was waking up to a big puddle of water overflowing out of the fresh water tanks vent...we did not fill the tank it was somehow back fed thru a one way check valve or thru the water pump which was off as we have full hook ups here. Does anyone know if there is a check valve in this line if not it has to be the water pump..
Thanks again, George