Heads DO stink...and not necessarily from the tank
Owners of raw water toilets on boats in salt water fight an ongoing problem with odor from sea water left to sit and stagnate in the intake and pump while the boats sit. Your profile indicates that your boat is in fresh water, but that doesn't rule out odor from the intake if your toilet uses lake water. So...Does your toilet use lake water or onboard fresh?If the toilet uses lake water, and the odor is coming from the bowl, there's a good chance that some animal or vegetable sea life was pulled into the intake where it's died and rotting in the intake line, pump, or channel in the rim of the bowl. The odds of pulling in something increase if there's a lot of vegetation in your waters, and especially if you anchor in shallow water. Or...I happen to know that the waters in several TN lakes are quite "organic"...owners of boats with raw water toilets on those lakes fight the same battle with intake odor that boats in salt water do.So here's what I'd suggest: close your intake seacock and remove the hose from it. Stick it in a bucket of water to which you've added a quart of distilled white vinegar...flush the whole bucketful through the toilet. Follow that with another gallon of water to which you've added 8 oz. of Raritan C.P. (bio-enzymatic toilet bowl cleaner that also destroys odor on contact). And, if the joker valve in your toilet discharge hasn't been replaced in at least two years, replace it. That should eliminate the odor, whether the source is the intake or the head discharge hose.