I don't think the vent is clogged
If it were, you'd have back-pressure from the tank when you flush the head and whole bunch of other problems you don't want to think about.If the odor is only IN the head, there could be trapped dead/decaying sealife in the rim of the bowl. Disconnect the intake hose from the seacock (close it first!), stick it in a bucket of fresh water heavily laced with C.P. and flush it through the head.Or it may not be the head at all...it could be the shower sump...for some reason, it never occurs to anyone to clean it. Again, C.P. will do a great job...its bio-enzymatic cleaners kill odor on contact and "eat" soap scum, hair, body oils ("bathtub ring" stuff) as well as clean out the primordial soup that grows in wet dark places--and STINKS.If neither of those turn out to be the problem, get back to me. We'll keep hunting till we find it and cure it.