How about creating a sweet tank? Our Catalina Morgan 440 comes with an aerator for the
holding tank. It operates on the same principle as one of those air
pumps for an aquarium. By blowing air in with all of the "nastiness," it helps block the organic breakdown that occurs in there, thus reducing the smell. It seems to work rather well, but I do recommend not running it while sailing. The
holding tank vent for the 440 is on the port side amidships and when underway with the
pump running, I've occasionally caught a whiff of that "sweet" smelliness. Ugh...