All vents black and gray have air admittance valves. Look under the sinks you will see them screwed to the top of a pipe. Sometimes you have to reach your hand in and follow the pipe up to the top. I had one not screwed in tight. Gave it a few turns smell gone. Had one go bad during a 3000 mile trip. Stop at Home Depot bought a new one smell gone They cost $6-$8 Easy couple minute fix Good luck
It will look something like this
https://www.amazon.com/Vent-Abs-Inli...ateway&sr=8-35
Jerry