On our 02 Georgetown, there's a city hook-up connector and a fresh-water hook-up connector, both regular hose-end hook-ups. The fresh-water tank doesn't get filled when we use the "city" hook-up at campgrounds. I suspect that, if your city hook-up is filling your FW tank, there's a cross-system leak somewhere, or maybe it's mis-plumbed.
Our city (Anacortes, WA) is one of the top five nationwide in water quality, so, regardless of the weight penalty, we always leave home with the FW tank full.
Frank and Eileen
No longer RVers or FR owners