Re: shower drain trap?
"P" trap keeps the nasty smells/gases from filling your living space with the lovely aroma of either stagnant grey water, black tank or park sewer... depending on what you are hooked up to.
We ended up using a standard cheap kitchen sink drain & strainer basket in our shower for two reasons. #1 It was cheaper than the traps I wanted. #2 It was cheaper than a shower drain. Going with a kitchen sink drain yielded two benefits. #1 allowed us to use a smaller drain pipe with out the need to use expensive reducing fittings. #2 I can close the basket drain and soak my feet (I do a lot of standing/walking on concrete floors). #3 We can drop the strainer basket in place and close off the drain while traveling, eliminating to potential smells that may occur because it's so hot outside the highway temps are causing the liquid in the holding tanks to evaporate which smells... bad. Yes, it does happen and part of the reason I never want to travel across Texas in July/August ever again.
It's your bus. Whether you do or don't use "p" traps is up to you. We did.