In all my years and 4 different campers, have never seen or heard of one that you could fill the FW tank through city water. Most of the ones I have seen have a valve that prevents city water from getting into FW tank when pressurized. Furthermore, can't imagine why that would be possible, as the city water supply would quickly fill and overflow the FW tank when turned on.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.