Fresh water tank overflowing when connected to city water

[Btsanders101]

We're in our 2018 Surveyor 243RBS. On the current trip we saw water dripping out from the cap covering the fresh water tank input. How is turning the city water on causing that tank to fill and overflow? Now we have the drain cap off and water continues to flow from it whenever the city water supply is active.

[HangDiver]

The check valve on your water pump is not seating properly, allowing city water to fill the fresh water tank. One thing to try:
1) Turn off city water at the source;
2) Turn on your water pump and open-and-close various faucets in your camper allowing the water pump to cycle on and off;
3) Turn off water pump, open a faucet and relieve water pressure;
4) Turn city water back on.

Never turn on your water pump when hooked up to a city water source.

Good luck