Ahoy msham421, No matter what type or equiptment or the boat it''s on the problem is still a basic plumbing/sanitation issue. That said, the physics and mechanics are fairly basic and straightfoward. I suspect the system you have will naturally over pressurize when the liquid volume approches full capacity. Inherent in decomposition is the build up of gas that must be vented off. Most any permeable material will allow the gas to escape when the pressure is great enough.The venting for such systems can become clogged or in some cases poorly designed. If your sure you have no liquid seepage and your only problem is odor; gas (what your smelling) is the answer. It is possible a leak in a vent or a poorly placed vent which allows odors to recirculate into the vessel could be the answer. Try a smoke test under pressure with an enpty tank and look for any leaks. Chemical deodorants can mask the odor at times but a port-a-pottie isn''t my idea of a good time down below. Big Red 56.