[bushpilot]

Gray water isn't nearly as foul smelling - so any that escapes via the drain would barely be noticeable.

Im really just kinda talking out loud - confirming theres not gonna be an issue.

The water in the p-traps on an rv sloshes around & I'm wondering if that might give odor a chance to escape via a drain.

Theres only 2 ways for a Black tank to vent (primarily thru the roof vent, out of the tank) and thru the bowl. Keeping water in the bowl prevents a seal (like a p-trap).

When i think of a COMBINED tank, I'm thinking they mix and are connected.

I believe a better way would be to (on demand) pump gray water INTO the black tank (when the gray tank gets full).