[Land Yacht]

Mike, typically only the toilet(s) go directly into the waste holding tank. The shower and all sinks/lavatories go into a pipe which terminates between the two valves on the driver's side. When the inner/manual value is closed and the outer/air valve is open, everything except the toilets bypasses the waste holding tank. IF the outer valve is closed and the inner valve is open, everything goes into the waste holding tank. IF both valves are close, the pipe that accepts the sink/lavatories starts filling up and at the top of the waste holding tank, tees off and dumps into the waste tank. That way if you leave both valves closed and run the sink, shower, lavatories, washer, etc, the grey water will fill the pipe between the two valves until it reaches a level above the top of the waste tank and then be discharged into the waste tank instead of backing up into the shower which is typically the next lowest point in the grey water drain system.

Thanks Michael, exactly what I was looking for. I had been told incorrectly by a RV repair guy,and it was not matching my experimentation with valves.