2 possible reasons
One is the on-board pump internal check valve is leaking allowing water to backflow thru pump into fresh water tank
The other way is if you have a water panel with valve/switch that you use to 'fill' fresh water tank, it is not fully closed or is leaking thru.
So if you 'gravity' fill your fresh water tank....pump check valve.
Sometimes using pump for a day or so can clean up the check valve (flapper) and then allow it to close.
Sometimes you can disassemble pump, clean up check valve and it will work just fine
Sometimes it's just easier to install an in-line check valve in pump suction line (about $10)
Is it time for your medication or mine?
2007 DODGE 3500 QC SRW 5.9L CTD In-Bed 'quiet gen'
2007 HitchHiker II 32.5 UKTG 2000W Xantex Inverter
US NAVY------USS Decatur DDG31