I am in the middle of dealing with this issue also
The vent is called an air admittance valve
I also let the trap dry out in that sink and lots of people chimed in it can be that too...
I did some serious flushing of the tanks last time i was there and made sure the sinks had a lot of water run through them
Also got this
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Seemed to be good when i left the trailer a week ago, will update my air admittance valve thread this weekend when i get back up to the camper, hopefully it will still be smell free....