Theoretically, if the black tank had more in it than the gray tank, and the gray and black tanks were opened together, the black tanks contents could end up in the gray tank if the drain hose didn't drain fast enough or had a blockage. That is not a pretty sight. LOL
And believe it or not, boondocking one time where there was a gray waste water collection thing, this guy was telling us how he puts on the termination cap and opens both valves so black tank runs over into gray to extend his boondock time. Not a bad idea IF you have "P" traps in all gray water drains but mine doesn't and I don't think most do. Shower drain usually doesn't and don't want to smell that. I think I did misunderstand your question at first. Yes sewer flush is just that to flush sewer. Maybe it should have better name of Black water(holding tank)flush.
