Fresh water tank overflowing when connected to city water........
Basically 2 ways
1) IF you have a water panel with valves then "FILL" is still open or leaking thru
2) On-board pumps have internal check valves to prevent backflow thru pump when on city water.....it is leaking. (TURN OFF CITY WATER___USE PUMP. Flow thru pump MAY clear debris/crud on check valve and allow it to close. When using pump if should not 'cycle' if check valve closes)
IF you have both.......which one is leaking?
Remove pump suction line at pump and see if water flows back out of pump with city water ON