Does your RV have a hose connection AND a fill port for the tank? If so, then you probably don't have a valve to switch from 'Fill' to 'City.' You might have a leaking check valve that is allowing water to fill the tank while on 'City' and after the tank is full, blowing out the fill port. Be careful, an over full water tank can expand and cause structural damage.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI