How do you fill your fresh water tank?
By changing valving/switches on a water panel where your cit water connects?
OR
Via a gravity fill port directly to fresh water tank?
Via a gravity port...then your on-board water pump check valve is leaking thru
Via the water panel.....then could be the 'fill' valve OR the pump check valve
Check pump suction side line/strainer for water flow....then you will know which one is leaking thru. Pump check valve or water panel fill valve