P traps under the sink prevent odor just like in your house.
I am not 100% certain, but am pretty sure, on a high wall the shower drain does not have a P trap, as it is right over the tank and there doesn't appear to be room for one. Someone correct me if I am wrong.
Bob & Doris
2009 Toyota Tundra CrewMax 4x4 - 5.7L - 6 Speed Auto - 4.30:1 2013 Rockwood HW296
I don't keep the valve open all the time i dump it once or twice a day i find it much easier to have them joinned because i mostly camp where there are no services so i use a portable tank that i bring to dumping station once every two days. i think the is a p trap in the shower. we never get any smell. But what the shower need is a back trap so when the gray tank gets full it does not back-up in shower.