Originally Posted by jimmyp
I had a jayco trailer that also filled the fresh water storage tank when hooked up to city water. The problem was my shurflo pump that had a bad check valve built into the pump.The trailer was still on warranty and the dealer gave me a brand new one.
Jimmy has this one nailed for you. The check valve in the water pump is allowing backflow to the fresh water tank.
You can try disconnecting the city water an running the water pump for a few gallons. A piece if grit may have been stuck in the valve. This may flush it out.
If the problem persists you can replace the pump. Or purchase an add-on inline check valve. That would be less expensive.
I would also recommend using a water filter for ALL water entering the camper. City water connection and filling the fresh water tank. The valves used in RV's do not tolerate particles or sediment very well.