Your pump has an internal check valve that should close when you connect city water if it is stuck open or not closing tightly then city water will back flow into the tank. Here is what I would do. with water in the potable water tank I would disconnect (not just shut off) the city water connection turn on the pump make pressure then turn it off and open a faucet to drain all pressure off. Then with the pump off I would reconnect the city water and open it fully to set the check valve. If that fails I would pull the pump and bench test it and go from there.
Jake my sidekick (yellow Lab) 10/04 - 05/20
2017 RAM 2500 CC 4X4 Cummins Diesel
2016 Flagstaff 26 FKWS
AF&AM & El Korah Shrine of Idaho