Is the odor inside the boat or outside?
Odor outside the boat is most likely coming from the tank vent line (which, btw, should NOT have a vented loop in it...more on that later). As tank contents shift in the tank, they displace air in the tank, forcing the air out the vent. If the air in the tank isn't just air, but smelly gasses, what comes out the vent will stink. However, you'd also have the same odor every time the toilet is flushed because incoming waste also displaces air the tank. If the joker valve in the toilet is worn, that smelly air can also escape back through the toilet. Joker valves should be replaced at least every two years, annually is better.That hissing sound from the toilet can be an indication that your tank vent is blocked. If air displaced by shifting or incoming waste can't escape out the vent, the tank will become pressurized and that pressure will get out anywhere it can...and back through the toilet is the line of least resistance. If you also hear the same hissing in the toilet during pumpout or while dumping the tank, it's 100% certain that your tank vent is blocked. Or, your odor in the head could just be from a shower sump in desperate need of cleaning...a wet dirty sump or bilge is a primordial soup that'll produce gasses that smell like a swamp or sewer...heeling causes the water in the sump to shift, same as in a tank, forcing the smelly air out the shower drain.Now...about vented loops in vent lines. While LOOPS (high arche)) are recomended in water and fuel tank vent lines to keep sea water out, a loop in the holding tank vent line is not recommended because the loop impedes the exchange of air needed to keep the tank aerobic and therefore odor-free. Nor are VENTED loops needed in fuel or water vent lines because vented loops are siphon breaks. Since tacking--raising the vent through-hull out of the water--would break any siphon started by sea water into the tank, vented loops in tank vent lines are a waste of money. Locating the vent and inlet tank fittings inboard on the holding tank inside of outboard will prevent sea water from entering the tank AND waste from running out the vent or back toward the toilet.