Onboard water system pumps water out the shore input

"gdbudlong".......The check valve should be just behind you're fresh/city water fill. It's there to do exactly what you're having an issue with. It closes the fresh water opening so that you're pump can build pressure and supply water to the rest of the coach.

Often, the valve gets debris in it and prevents it from closing completely. As stated above, sometimes it can be cleaned out or even whacked to get it to close. If not, you'll need a new one. They're inexpensive, just a pain to replace on some coaches.