The tank isn't the culprit
Why does everyone automatically assume that the holding tank has to be the source of any "sewer" odor inside the boat? 'Cuz unless the tank is leaking, odor from inside the tank has only one place to go--out the tank vent line. So--unless the tank is leaking--it CAN'T be the source of odor INSIDE the boat...the odor has to have a source INside the boat. Permeated hoses are one source...the only cure: new hoses. A wet dirty bilge is a dark stagnant swamp where micro-organisms in sea water die and decay, and where the same molds, fungi and bacteria that grow in all swamps grow...and STINK. A wet dirty bilge can smell just like a sewer. The cure: clean the bilges...really CLEAN 'em and flush all the dirty water out, don't just throw in some bilge cleaner.Dirty sumps are another source. The cure is the same as for bilges.The chain locker is another source...it's another dark wet breeding ground for critters that stink.The cure: pull the whole anchor rode out and lay it on the dock...hose it off with plenty of clean fresh water and leave it in the sun to dry while you clean the chain locker.Seawater left to sit and stagnate in the head intake while the boat is closed up is another source of odor. Flushing the toilet a few times to clear the stagnant water out cures it...my book describes a simple and inexpensive way to prevent it...along with instructions and tips for maintaining the sanitation system, replacing hoses, choosing a new toilet, and preventing odors.