a beautiful boat should sail forever.
new 30 gallon holding tank from Marinesan. You send them the plans for what you want, and they build it. This layout follows the diagram on page 544 of Nigel Calder's essential book Boatowner's Mechanical and Electrical Manual, third addition, which greatly simplifies the holding tank system.
The right angle 1.5" fitting is the intake from the head, the straight 1.5" to the right is the dip tube/pump out. The 3" view port/cleanout id in the center, and the two 3/4" right angles are the vent tubes.
I added an extra vent based on the information that air is essential to no odor and liquification - so with one vent on each side of the boat hopefully the pressure differential from one side to the other will encourage constant air flow.
There is one more 1.5" fitting on the bottom of the tank, DIRECTLY UNDER THE DIP TUBE, that connects to a seacock/thru hull for dumping at sea. This position allows for a stick from the deck to go thru the tank and seacock in case anything gets clogged up.
After shipping, hose, and fittings the system is approaching $500. That does not include the head itself, which is your basic manual Jabsco somewhere between $150 and $200. This is a keep it simple stupid system (KISS), and robust, I hope.