Yes, several other owners have experienced this, there is a bypass valve that seems to stop operating, allowing the fresh to enter the tank.
There is a check valve that is incorporated with the Shurflo pump to stop water from back feeding into the freshwater tank when hooked up to city water, but if this is bad wouldn't the tank continue to fill and you would have water continuously coming out of the tank overflow? J&G didn't indicate this was happening. ??
Hybrid Mode
