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.
Peggy Hall (known as the head mistress) has been the go to on this subject as far back as I can remember (late 1980's). Here's another link to a source for her excellent book - The New Get Rid of Boat Odors
Do you have a stinky Head?
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