We have two 30-gallon gray tanks slung under the Overlander, each with a standard unthreaded opening for a black ABS pipe. On the galley side, it's easy enough to plumb the kitchen sink down to the tank with a normal ABS "S" trap. On the head side, I'd prefer to avoid raising the shower floor any more than necessary (to allow room). Even squeezing in a shallow "P" trap will force me to raise the floor.
These are only gray water tanks so the issue is slightly different than in a household. On the other hand, having some kind of water (or mechanical") seal does afford certain advantages. Any thoughts, master plumbers?