These air admittance valves are supposed to allow air to come in but not allow the air to enter the coach. They do fail. Most are cheap and some are better and more expensive. I saw these available at Lowes a few years ago. Should be able to find them at most larger home improvement stores.
Chris & Maria
2012 36RL, Full Body Paint, Dual Awning, Dual a/c, Splendide Combo, Fisher Paykel dishwasher, 6.5kW Onan, Goodyear G614's. 2015 F550 6.7l, 4X4, Lariat, CC, 176" WB, Long Box, B&W Companion