I just returned from chartering a Sunsail Beneteau 473 in Croatia. Beyond learning that Croatia is wonderful and the sun shone every day and humanity should never have migrated away from the shores of the Meditteranean, Sunsail has installed in some of their boats a holding tanks system that actually WORKS! They call them “gravity tanks.” It is so simple to use that.. you actually use it, and as a consequence the water in their huge marinas is so clean you can swim in it. At the hull in an easy to access location (inside a locker near the shitter) is a throughull fitting that is huge- I have no idea what goes on inside this thing. On top of it is a big red handle. When the handle is shut, the shit goes into the holding tank when you pump the toilet. When the handle is turned 90 degrees, two thing happen… the contents of the toilet go directly out to the sea when you pump it. And, the contents of the holding tank drain by gravity out to the sea. It’s as simple as that. And on our boat with three toilets, there was never any smell anywhere. It is simple, there are far fewer hose-clamped connections than any other system I am familar with, and did I mention, it doesn’t smell!