All of the above are suggestions to cover up the smell. Also, even a small amount of moisture will ruin the vent filter.
The only way to eliminate the odor is through proper ventilation and keeping the contents of the holding tank aerated.
Copied from a previous post by Peggie Hall:
"For about the price of 3 filters, you can solve the problem permanently with the Sweetank system.
Filters only last about a year, btw, and are toast immediately if they get wet. They also help to create the problem they're sold to solve (why couldn't I have invented something that does that???) by restricting the exchange of air through the vent needed to keep the tank aerobic.
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Peggie Hall, Moderator
Specializing in marine sanitation since 1987, Author: "Get Rid of Boat Odors--A Guide to Marine Sanitation Systems and Other Sources of Aggravation and Odor."
http://shop.sailboatowners.com/boat_odors/
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Old Hickory Lake
2002 - 16 X 70 Sumerset