Plumbing Question

Most likely. You can purchase one at most big box or hardware stores. Here is one from HD.

Seems to me that that vent should go up and vent out through your roof. Ours does.

What you have is called a, "cheater vent."

I agree with Ron that's the culprit. It's only supposed to work in one direction to allow the plumbing to properly evacuate liquids and yours is probably faulty. Hopefully the RV mfgr. installed it properly and it will still unscrew from the pipe. When you install the new one, go ahead and use some Teflon tape on the fitting to ensure the threads are sealed. If the mfgr. didn't do that, I suppose it's possible the odor is getting past the threads, but I'd just replace the device and use the tape to make sure.

I agree with Ron that's the culprit. It's only supposed to work in one direction to allow the plumbing to properly evacuate liquids and yours is probably faulty. Hopefully the RV mfgr. installed it properly and it will still unscrew from the pipe. When you install the new one, go ahead and use some Teflon tape on the fitting to ensure the threads are sealed. If the mfgr. didn't do that, I suppose it's possible the odor is getting past the threads, but I'd just replace the device and use the tape to make sure.

It's called an air admittance valve. They are used extensively in both the RV and home industry when it's impractical to run a vent to the roof. An example in a home would be under an kitchen island sink. They are not "cheating". It is a building code approved vent system. However, they do fail from time to time. Just purchase one from any big box store. They are threaded in. It is not normally necessary to use any type of thread sealer because the pressure in the system is very slight. However, I'd personally use Teflon tape on the threads as mentioned above.

FWIW - I had one fail in my brand new fiver under the bathroom sink and it was a very easy fix. Replacement took about five minutes.

Yep, I have 2 in my RV. I have replaced them both over the course of 7 years, and have a spare in a basement for the next time. They only cost about $6 if I recall correctly.

They are to vent the drain where they are attached to keep the water from being siphoned out of the U trap.

Even when those things do work, they don't work well for odor when it's hot out.
I'd just replace it and hope for the best. If you still have the issue then I'd just dump some toilet chemicals down there to get rid of the smell when it's an issue.

There are some bottom line AAV's that don't seal well.

I got a Pro Vent PV-140 to replace mine at my local plumbing supply. Cost was under $5. Make sure that the one you get has a spring to insure a positive seal.

Ok, thanks.

So, they are a single unit and just screw off the end of the pipe?

Yes, they just unscrew. BTW, I've replaced mine once with no change (still get occasional grey tank odors). We have this valve plus the tall vent stack to the roof. This spring I'm installing these to see if things get better. RV Siphon 360