Some background and some comments. Had a Nature's Head in our Tiger. Our XPCamper came with a cassette and, after one season, we ripped that out and installed a C-Head. Why the change of brands? The C-Head is shorter front to back and the shower space in our XP is very small.
Now the comments:
-- As long as urine is kept out of the feces, there is no real odor and what there is is not objectionable. This is NOT the case with outhouses and long drop toilets at campgrounds as they mix the two and thus they depend on doors and chimneys to limit odor.
-- The greatest danger to a composting toilet in a vehicle is the entry of shower water. (And, of course, drunk Uncle Louie taking a whiz in the wrong place.) Sadly, neither of the two toilets that I am familiar with does a great job of keeping out the water. But both work as long as the seals are in reasonable shape and you recognize the things that will cause water to enter. In the case of the C-Head, this is water on the deck that flows in when you open the toilet to remove the urine bucket.
-- While C-Head maintains that venting is not required, I believe that it is almost essential to control moisture, whether from condensation or shower leakage. Venting is not really a requirement to control odor because there simply isn't much. Not even when in use if your fan is strong enough to actually create negative air flow.
-- We have solar powered vent in our bath and it was set on exhaust to dry the shower, etc. It was not strong enough to pull odor out of the toilet. A larger vent might be able to do that. I haven't tried reversing it to pull air in as it just died on this trip! ;-(
Conclusion: I believe that you should vent your composting toilet with an active or passive vent. You don't want to seal the toilet as you want it to be able to pull air from inside the vehicle and exhaust it.
As always, YMMV.
P.S. Cassette toilet available for free, pick up in Nevada!