The factory installed valves on under sink vents on most MH are of poor quality. Replacements can be bought at Lowe's for $3.95 for quick fix and may last longer than the factory valves. I had the expensive studor vents on hand so I used those to get them out of my garage. They sure ended my odor problem.
Those with factory valves should carry one of the Lowe's valves as a spare as it is not fun when one of the factory valves hang up.