You might want to consider this instead of a "composter"
SeaLand Traveler at Defender Self contained...has the same bowl as the VacuFlush...uses onboard pressurized fresh water and so little of it that the 9 gallon tank holds as many flushes as a 40 gal tank connected to any marine toilet. No moving parts to maintain, no plumbing except the flush water inlet line, pumpout line and vent line...and with a y-valve in the pumpout line a macerator pump can be added to dump the tank overboard. Costs a lot less than a "composter too."
Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein