I am not so sure about this.
There is a check valve (that physically is in the 12 volt water pump) that prevents city water from getting into your fresh water tank. To replace that check valve you would need to replace (or rebuild) your water pump.
If the check valve has cracked or the seals hardened such that it is a slow leaker, it is "possible" that it filled it over months of city water pressure, but at some point it would be "full" and you would have water coming out your fresh tank vent (next to the gravity fill port).
Having said that, are you sure you never put water in that tank?
The fresh water drain is a TINY spigot for a 38 gallon tank.
Even half full, it takes about 30 minutes to drain completely.
I usually just open it and go about my business of getting it ready to winterize. By the time I am done winterizing, the tank is usually empty and I close the sigot.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW