RV Water Pumps have had internal check valves for years....many years.
Could just be debris holding the check valve (flapper/diaphragm) open.
Installers don't cleanup after cutting holes/installing fittings etc in fresh water tank.
Suction strainer on pump should catch junk but doesn't always happen.
Use your pump.........run a full tank thru it.
Sometimes that will clear pump and allow check valve to function.
Course you will need to clear aerators on faucets.
This of course is IF you have a gravity fill for fresh water tank and not a 'fill' valve on a water panel. Fill valve could also allow fresh water tank to overfill when connected to city water if open or leaking thru.
Depending on how much water you had in tank.....filled to overflow in few hours is pretty quick for a leaking check valve.
From one 'salt' to another....THANKS! for your service.
USN.....Machinist Mate E4/E5/E4-E3/E4