There is a check valve in the water pump. The water pressure from the city water line is pushing this open to let water into the freshwater tank and over flowing it. Shut city water off and for now use water out of water tank.
1) Do you have a water pressure reducer on a hose from the city water to the RV..... A BiG must, as all campground and city water pressures can be very high and blow out your RV water lines....
2) There also may be some type of debris at the RV water pump check valve and keeping it slightly open. Shut city water off and use the water pump and pump water out of the fresh water tank. This may flush out the debris. If it does not flush it out, you may have to remove the check valve and clean it out..
2013 Forest River Salem Hemisphere SBT312QBUD
2001 CrewCab F-250 7.3 PowerStroke Diesel
SuperChip, BTS transmission, 6.0 Trans Cooler