On a boat that size, you may be better off with
a permanently installed, fitted for pumpout, portapotty. 'Cuz if the toilet is still original, it has to be worn out by now--or so obsolete you can't even get a rebuild kit for it any more. The cheapest toilets don't even have to be that old. And a holding tank big enough to be worth the effort and expense to install will use up a lot of storage space...because of the difference in the amount of flush water used a by a manual toilet, you'd need at least a 15-18 gallon tank to provide any noticeable advantage over the number of flushes a 5-6 gallon "pumpable" portapotty holds. My choice would be the SeaLand 965-MSD. No plumbing required except pumpout line and vent to the outside of the boat. You already have a discharge thru-hull, so if you want to be able to dump it at sea, you'd only have to install a y-valve in the pumpout line, with one side going to a pump and that thru-hull. A clean neat, self-contained installation that offers all the advantages of a marine toilet and holding tank, but with none of the plumbing and maintenance they require.
