water over flowing

I remember reading some time ago of a fault in the check valve on the tank side of the fresh water pump. A large batch were bad and were installed on RVs. If the check valve fails it will allow a flow (not a strong flow)through the freshwater system, through the pump and into the fresh water tank. You should have a valve between the fresh water tank and the check valve/pump connection. If you have this valve, close it and see if that solves your overflow problem. If you don't have this valve, then the check valve is going to continue to leak.

Mine did that and I noticed that I had the selection handle on the mark for city water and it was actually slowly filling the tank to over flow. Started moving the handle past the city till it stopped and that shut the fill off completely. Now I don't pay detail attention to the decal location, but rather the feel of valve/handle.

Thank you all. wrg4s50s I think you have it as I have no valve to leave open . mine is one place for city and one for tank.
Again thank all

Another way of water back filling in your fresh tank is the backflow valve in your 12v water pump going bad. As you stated, may just be some trash stuck in the pump. You can disassemble and see, or replace the pump, or install a backflow valve to the outlet side of your pump hose. There is a product called sharkbite that is very simple to install on pex lines if that is what is used out of your pump, but there are numerous options. They are available at most hardware stores.

The valves have O-rings in them and when one or more of them gets worn, or gets a nick that allows it to leak, water can feed in to your fresh water tank no matter which position the valve is in. You either have to replace the valve or rebuild it IF it can be rebuilt.

Mine would do that even when the valve was in the correct position. Had to replace the valve assembly and it works as it should.

It sounds like you left the valve to to fill position on or partially on.

Make sure your Tank Fill is off.