pchast wrote:we are not just talking about gravity. With travel that water seal will splash out of
the trap... Would the '-waterless trap' work in th at situation?

What is the RV standard in this situation?
Most RV's I have seen have the P Traps and no issues. Really, it would take a whole lot to splash the water out to the point of loosing the vapor seal. Not much to worry about. In any case, it is gray water... wouldn't be stinky like the black water... which has the toilet sealing it up with no P trap. The toilet is a direct drop in. With gray water, that is tough to do with a bathroom sink, shower and a kitchen sink in different locations in relation to the tank.
Larry, if you were not planning on a P trap.... you just need to build the floor for the shower up a few inches and then you can offset the plumbing however you need to drop it into your gray water tank or onto the ground or external/portable holding tank. (however you are doing it). That should solve your problem in any case and maximize headroom. Also, if you need the headroom, you could put a nice skylight directly over the shower... that is what they do in a lot of trailers to gain headroom in the showers after they build up the floor to accommodate plumbing.
