Water leak only when connected and using city water..

What kind of pressure regulator are you running in your water line? Campground water is OFTEN well over 100 pounds which is WAY more than the 40 or 60 lb.on newer models) RVs are designed to handle.

Yep! You got a water leak somewhere under the main floor and the sub-floor. Take the cover off where it goes into the coach to expose the water lines. Plastic lines with plastic connections. Probably one of the places where it connects might have come lose or broke.

Are these separate connections. Maybe the receptacle needs a new washer. If not, You may need to run your line from the connection to the entry point to see where the leak is.

I don't know about your particular unit, but in most, the city water comes into the system though a check valve. So, what you have is a plumbing leak upstream of that check valve. It is probably (usually) the result of someone winterizing the unit by using only the potable pump and failing to blow out the city water connection.

Matt