[Chuck v]

It should not be possible under normal circumstances...

But, If both gate valves were open AND the drain hose itself were blocked/capped then maybe both tanks could mix. This is extremely unlikely for you to have done, and quite unsafe from a sanitation standpoint. There is no other plumbing failure that I know of that would result in grey water moving into the black tank.

Some coaches allegedly have the bathroom sink drain into the black instead of the grey tank to try to make up for users not running enough water along with the flush, but I have never confirmed any instances of this in a way that I believe it. Maybe other members here have different stories to tell...

Chuck