My wife calls them foo-foo valves...lol
Its called an Air Admittance Valve among other names. I would imagine that most RV or plumbing supply stores would have them.
Some screw on, others just slide on, so if you change yours, you might want to remove yours first.
2014 Keystone Laredo 292RL
2013 Palomino Maverick 2902
2018 GMC 3500HD, 4x4, 6.5' box, SRW, Denali, Duramax, Andersen
DeeBee, JayBee, and Jed the Black Lab
The hurrier I go the behinder I get. (Lewis Carroll)