Black tank overflow

I didn't read all responses, but concerning your fresh tank unknowingly filling, do you normally fill the fresh water tank with the same hose hookup as your city water supply? Is there a valve that directs water to either fill your tank or thru the plumbing system? If so, the water pump on our MH was letting water backflow thru the check valve on the water pump when the fill valve was closed. I put another ball valve in the plumbing immediately after the pump and close it when we don't use tank water...The idea might need a one line drawing? This would have nothing to do with the blackwater issue.