SWAG, any chance it's the grey tank air admittance valve?
Sorry, I don't know your trailer setup but why not look under lav sink for a black cap thingy on the sink drain line somewhere near the high point. It's to let air in and not let stinky out from grey tank. Mine unscrews, inspect check etc.
You can search air admittance valve to see what it might look like.
2005 Dodge Ram 2500 5.9L 6 Speed
16" Michelins, Hi Spec Wheels, Max Brake, Dexter 4 Piston Disc Brakes, Carslile Actuator, Equal-I-Zer, Dill TPMS. Campfire cook. BMV-712