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. By reducing the vacuum, the tank and hoses will last considerably longer and be less prone to operational failure. Since the toilet discharge will be anti-siphon protected, it can be located anywhere, either above or below the waterline.