It prob'ly isn't your tank
It could be your sanitation hoses have permeated with odor. To find out if that's the culprit, wet a rag in HOT water...wring it out...wrap it around the hose. Leave it there till it cools...remove the rag and smell it. If you can smell the same odor on the rag, the hose has permeated and must be replaced. Test every section of hose, and use a different clan rag on each section.It's also possible, unless your tank is very thick-walled, that it's become permeated...test it the same way you test the hose...plaster the rag on a vertical wall. If the tank is aluminum, there's a good chance that it's leaking at a seam or a fitting if it's more than a few years old.Or...you may be leaving sea water to stagnate in your intake line and in the channel in the rim of the bowl. There's a very good fix for that in the article "Head Intake Odor" in the reference library in the Head Mistress forum.However, the sanitation system isn't the only source of boat odor. A wet bilge is a dark stagnant pond that breeds bacteria, molds and fungi which emit gasses that smell like a swamp (which, technically, it is) or even a sewer. Same is true of sumps. If you haven't cleaned--really CLEANED: scrubbed or power-washed with a heavy duty detergent bilge cleaner, and then rinsed till the water coming out is clean enough to drink--and THEN dried it out completely--in more than a year, I'd bet money that's at least part of your problem...'cuz you'd be amazed at how many calls I get from people who've already replaced everything in their entire sanitation system without any improvement...but cleaning--not just throwing some bleach and bilge cleaner into the primordial soup, but really CLEANING it--the bilge got rid of the odor completely.