360 Siphon sanitary roof vent

We have a rotating type but I agree that they have little value...with one exception....that is if you "flush in motion".

It seems that it prevents down pressure from the movement down the highway from pushing air back down the vent stack.

Otherwise, I have not ever had a smell when sitting still..other than when someone forgets and doesn't turn the fan off when flushing..

I have to agree with bemmerphile1. What you have here is a solution to a nonexistent problem. Thousands of RVs work fine without any additions. If you are getting odors from the black tank then you have a plumbing problem that should be addressed OR you are flushing with the vent fan running. In that case you will pull odors from the tank.

I did read that the water in the p traps can empty while driving.
But that doesn't account for the black water tank smelling

You cannot siphon air from an otherwise sealed system. All of those products are essentially snake oil. If you have odors anytime other than the brief instant when flushing, you have a plumbing problem that needs fixed.

We had the rotating type vents on our tanks for a few years until a branch clipped one and bent the shaft. I replaced with the 360 Siphon and IMO it works better.

Our 360 works just fine. As did the one I installed on our previous coach. I like that it has no moving parts and a lower profile than the "snorkel" style rotating venturi vents.