Manufactured housing builders sometimes use a mechanical vent rather than vent to the outside. You will find the mechanical vent under a sink just kinda sticking up there by itself. It will have a top on it and slots in the sides of it, there is a spring mounted stopper inside it that allows the discharge to pull air in behind it to keep down the gurgling. The spring rusts out and the vent stays open, allowing odors. Normally you can just unscrew the mechanical vent, take it to a mobile home parts house and get a new one.