There are two paths from the city water inlet to the fresh water tank on most RV's. You know the obvious one and you positively blocked it.
The other one is the water pump. It has not one but two check valves and if they fail.. What you get is what you got.
To test.. You need a towel.
Turn off the water outlet (Or drain the fresh tank) open the pump-inlet strainer and lay a towel under it to catch any water that escapes when you open it.
now change to a dry towel and come back every hour if towel is wet. valve is leaking.
Two fixes. you already know the first. second is new pump.