I'm specifically looking at this listing.

https://www.amazon.com/gp/product/B00SQ1K19A/ref=ox_sc_act_title_2?smid=ATVPDKIKX0DER&psc=1 - https://www.amazon.com/gp/product/B00SQ1K19A/ref=ox_sc_act_title_2?smid=ATVPDKIKX0DER&psc=1

We do intend to use the wet bath exclusively at camp sites. So that might push our usage up a bit more than most as I gather most people tend to try and avoid the use of the wet bath when possible. Part of the reason we purchased the trailer is to avoid public restrooms in the time of COVID. But still, we've got no smell to force the use of this vent right now.

Y'all are convincing me that I should probably just cool it and wait until I have a problem before I try and solve it. 

As my daddy always said, "if it ain't broke don't fix it".

I have had smells in my wet bath, as have many others. The usual cause is letting the traps dry out from lack of use. That won't be your problem with 4 people using the bathroom. Be sure to wipe the bath down and dry everything out after showering and get any wet towels etc out of there, otherwise it will stay damp all the time and grow mold.

If you do the math on how much positive pressure at the top of a vent stack from the wind it takes to push air past the water in a trap its very unlikely that that could ever happen. Direct Cat 3 hurricane winds still exert less pressure than a one inch water column. Traps are designed with at least 2 inches of water seal.  In my house on the NC Outer Banks I never once got a sewer smell even in the strongest blows. 

+1 on the dry shower trap. Much of the "sewage" smell is actually coming from the gray water tank through a dry trap.

One other thing--don't run the vent fan when you flush. It will pull stinky air from the black water tank since there is a straight shot to the tank and no trap. That is the same as the traps in the sink and shower. The fan will pull stinky air from them if you don't have enough windows open. It will pull from the source with the least resistance.

I have gone weeks with 1/2 full black tank and never get a smell. You might want to wait and see. Now one thing that makes a difference is your septic treatment. All treatments are not created equally. My personal favorite is from thetford, it's called Aqua Kem. It's a thick gooey blue liquid. It costs a little more than some others, but, I think it's well worth it. I tried 3 or 4 different types (with bad results, odors) before settling on this one. Others will recommend other things, but this is what works for me. 

Good point on the toilet ball valve. That could conceivably leak stinky air and cause a smell since there is no water seal, although I haven't seen any reports of that on rpods. The swivel roof vent might arguably help with that but personally I'd rather replace the ball valve if that happened so there was a good seal there. 

As for the ceiling fan drawing air through the traps, I assume you mean if they are dry. There is no way that little fan can produce enough pressure differential to displace a couple inch water column in a trap. A hurricane won't even do that. 

We agree that the traps would have to be dry for the fantastic fan to pull air up that way.

I got that you can get stinkies for the few seconds the ball valve is open. If the valve itself were to leak air (which is possible but I haven't heard of it) then that would be a continuing source until fixed unless the black tank is kept at negative pressure relative to the bathroom, which the roof swivel vent would presumably help do. A faulty black tank flush valve could conceivably cause an issue. In either case, I'd rather smell it so I know to go fix it than have a swivel vent concealing the problem. 

One other consideration is the seal between the toilet and the floor. 'm not sure if there is something like a wax ring there. If that is not sealing properly, stink could be pulled through there.

- Dry everything after a shower (already doing that out of pure perfectionism.)

- Cut the fan before flushing if no windows are open.

- Generally try to have windows/door open if the fan is pulling.

The description of where the fan is going to pull its air from is illuminating. I had not considered all the places it could potentially be pulling from and what that might do in terms of smells.

In any case, not gonna replace that vent anytime soon. We'll just enjoy things until a problem arises.