My fresh water tank was recently filling also while on city water. After 2 1/2 yrs. of religiously turning the Anderson valve to normal, after disconnecting from city water preparing to travel and use the pump in transit, this one time I neglected to do that. While in transit, the pump worked fine in the city position. When I hooked backup to city water the fresh water tank started overflowing within the hour. From previous experience, I figured it was the built-in check valve in the pump so recycled back and forth between city and pump/tank to try and free the check valve - no luck. I decided to change the Anderson cartridge because that was an easier place to start and as it turned out my old cartridge was very dry and hard to turn so the change out was good but didn't solve my problem. I had already upgraded the water pump to the Shurflo 4008 some time ago because it is better and quieter than the original Remco pump. I figured it was old enough to justify replacing it again and now everything is working as it should. I will continue to put the Anderson valve in the normal position when using the pump in transit
to avoid this problem again.