Most of the older Maxums don't have charcoal filters on the holding tank vent lines. If there's much in the tank, they can really stink. The simplest fix is to put in an aftermarket charcoal filter on that vent line. It's a nasty job, but you only have to do it once. Make sure you put it in a place where you can easily (and quickly) change the charcoal if you have to.