Welcome to the forum. As a suggestion, you need to edit your profile to indicate what coach you have. You can add the info to an avatar that shows up at the bottom of each post you make. It will tells us what year make and model RV you have. We can't help you much if you don't tell us what RV your talking about.
A guess, is that you have a valve, like most RV's, that needs to be moved from open to closed. In the open position it adds water to your tank, in the closed position, it supplies water to the coach. The valve usually says "City Fill".