Whit - The holding tank I installed is a Sealand 401500 (original number). It is 12 1/2 X 22 X 19 inches, but has one long edge cut off to make it a bit triangular. It is 15 gallons, and the new Defender catalog that arrive last week has it. It fit perfectly beneath the port forward berth, just forward of a drawer (with a small locker beneath). I only had to add a lateral brace across the front of the tank to hold it, since it nestles between the hull and base of the berth tightly. Running plumbing to the head was straightforward, and I have two Y-valves and a Henderson pump along with a pumpout port on deck.
There is a pumpout boat in Quisset that will empty it during the week when we are not on board, and when we are far enough out we can also pump it overboard. The sink in the head drains into the toilet, and we usually pump gray (and other minimally colored) water directly overboard. But since we regularly swim off the boat we are pretty scrupulous about not discharging solid waste.
Rick