HELP! Fresh water tank filling when valve set to “City”

I have the same problem and it is the check valve in the pump. For a bandaid fix, a dealer put a cutoff valve before the fresh water tank (under the bed in my case) to keep the city water from filling the tank. I do need to replace the water pump or rebuild it, in my case, since I have replaced one before in another unit, I will just put a new pump in. My biggest issue, is when I use the pump, I have to turn it off each time as it keeps pulsating even if no faucets are open. Guess it is backflowing into the tank.