John gave you good advice, 'cuz in the 20 years I've been solving odor and other marine potty problems on this and other sites, I have indeed written volumes about stinky tanks and how to prevent 'em from producing odor--two books in addition to all those forum posts (see my signature here for a link to the latest one) And I'd have thought that Granger had learned enough about it, by osmosis if not need to know, to offer better advice than "move the vent."
Seriously...you CAN have an odor free boat with no odor out the vent. Search for "holding tank odor" and/or "odor out the tank vent" for a good head start on solving the problem ...and I'll be glad to give you the specific help you need to accomplish it on your boat if you'd like to send me a private message.