First of all, if your boat is in US waters, I believe the holding tank is legally required. I would plan on pumping out the tank a bit more frequently than what you're suggesting, as from your description, if the tank is only infrequently used, it may be a long while before the tank is "full". Also, waiting for the tank to be "full" is just asking for trouble.
BTW, pumping the tank out through the seacocks is illegal if you're within three miles of land, and even further in some cases with NDZ.
You'll also have to design the system to allow the seacock to be locked in the closed position and the output of the head to be locked in the position of draining to the holding tank, as I believe this is a requirement for USCG inspections in NDZ areas.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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