As Tom says, it's possibly a check valve. In my coach the check valve is part of the assembly where the city water hose screws into the coach. That hose fitting on the coach, along with the flange all come off, and on the inside of the fitting is a spring loaded stopper which prevents reverse water flow. Sometimes when the coach is not used, the corrosion causes the stopper to get stuck. If you remove the fitting, and see the little stopper assembly, you can manually loosen it and by working it a few times it should remain unstuck. If not, and if that's the problem, that type of fitting is a common replacement at an RV supply store.
Of course, look for one of those pesky filter screens too.
If that doesn't work, let us know and we'll try to figure it out together.