Water pump check valve VS City water pressure
I understood you from the get-go, since I have the same issue and we are not alone. Here is the issue, the "check valve" on our trailers is located IN the water pump, when you attach city water that has too high pressure, it will overcome the valve and fill you fresh water tank. So, what is too high pressure, it varies, mine is between 45-50 PSI (I use an adjustable pressure valve just for this reason). I have read that disconnecting the city water, turn on the pump, draw water from every faucet will "reset" the check valve, also that when connecting the city water make sure the pump is off - since I have not tried either of these, I have no idea.