Water seeping from city connection to fresh water tank

How do you normally fill fresh water tank?
Gravity fill port or a 'valve/switch' you turn to FILL

Gravity port...then pump internal check valve is not closing and allowing backflow thru pump when using city water. Typically you run pump/flow water thru pump and clear the debris etc BUT you use pump frequently so probably need to replace internal check valve in pump

'Fill' valve/switch...
Leaking thru/bad o-ring --bad valve
OR pump internal check valve