Plumbing Question

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.