The air admittance valves are used when there is no roof vent close by.
They keep the water in the trap from being sucked out when you dump. They are inexpensive and usually easy to replace. They unscrew and are usually hand tight.
Clay WA5NMR - Ex Snowbird - 1 year, Ex Full timer for 11 years - 2004 Winnebago Sightseer 35N Workhorse chassis. Honda Accord toad.