Interesting question, Phil. I just took a crawl under my TM to check it out. There are no household-style water traps (P-traps or U-traps) in either the gray water or black water dump lines. And the outlet from the toilet comes straight down from the toilet to the external dump valve - no water trap.
I'm not familiar with code for RVs, so I won't try to quote it. These are my own thoughts on the topic.
The trap function (preventing sewer gasses from rising into the living spaces) doesn't have to be performed by a water trap. In the case of the toilet, I suspect that the slide valve built into the bottom of the toilet serves the trap function. The seal, being waterproof, is really quite tight.
In the case of the gray water, I seem to recall that there are traps upstream from the holding tank. In other words, there is a water trap under the kitchen sink, and one under the bathroom sink. I just don't know about the tub, and I can't see up there. But since the sinks have water traps, it makes sense that the tub has one as well - otherwise there would be no point in installing the others.
By the way, there is another reason not to leave the slide valve under the toilet open. Go to the TM Reference Library, read the tutuorial on the toilet, and look for the phrase "black cone".
Bill
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