We had the exact same thing happen in June, hooked up in park, water overflowing from fresh tank. After some thought, figured, like you did, that the check valve in the pump failed, which allowed water to flow to the fresh tank. Checked the area where the pump is, went to a local hardware store and picked up a 1/2" check valve, along with two sharkbite adapters, cut into the line feeding the pump and installed the check valve, and had no more problem. In the photo, the check valve is in the lower part of the photo, the power fill valve is above it, and you can see the white hose that goes to the fresh tank.