I've come across a Type III MSD holding tank system the author calls "KISS (keep it simple system) that I'd like to get your opinions on, it seems the best thought-out head and storage tank layout I've come across.
From the web site:
The concept is simplicity itself. The tank, installed above the waterline, is emptied overboard by gravity. All waste goes through the tank. The toilet pumps directly to the top of the tank.
(MSD) there is no "Y" valve or any other extra leak-prone joint in the system. The toilet itself can be either manual or electric, it makes no difference.
The outlet at the bottom of the tank leads to a "Tee." One of the fittings goes to the seacock, the other to the deck pump-out fitting. To use the tank, you close the seacock. To flush straight through, you open the seacock.
No Complicated pumps or valves, no smelly leaky vented loops, yards less of piping, dozens less hose clamps and no electrical connections necessary.
The K.I.S.S. system is elementary. The toilet pumps the waste up above the waterline into the holding tank. Gravity pulls it out. Simply by closing the outlet seacock, the holding tank is in use.
If your are emptying it offshore, you just open the seacock and it drains out by itself. To rinse, just pump the head.
Another important aspect to this system is that the tank is higher than most, especially those way down in the bilge, thus reducing the amount of "head" (suction) needed to pump the tank from dockside pump-out facilities. By reducing the vacuum, the tank and hoses will last considerably longer and be less prone to operational failure.