Two good choices:
The Dometic (was SeaLand) 711/M28 Marine Traveler is one good possibility. It doesn't appear that sbo.com online store has it, so I won't get shot if I post a link to it on Defender's site:
SeaLand Traveler at Defender
Only one thing they leave out: the tank footprint is about 20" x 20" which makes it too large fit in some head compartments.
If it won't fit, an "MSD" portapotty is another good choice. Not as elegant as the Dometic Marine Traveler, but it costs about half as much. The "MSD" designation in the model name/number means it has fittings for a pumpout line and vent line, and is designed to be permanently installed (actually just sturdier brackets than portables, so you could still take it off the boat if you absolutely have to), which means that although it's still called a PORTApotty, you don't have to carry anything off the boat to empty it.
A 5-6 gallon model holds 50-60 flushes...you'd need at least a 30 gal tank to hold that many from a manual marine toilet. No plumbing needed except a vent line and pumpout hose--so no new holes in the boat...and -0- maintenance needed except for rinsing out the tank--which you can do with a bucket while it's being pumped out. Total cost including the pumpout hose and vent line is about $200--a fraction of what you'd spend for toilet, tank and all the related plumbing needed. And the best part is, you have all the advantages of a toilet and holding tank without giving up a single square foot of storage space.
The Sbo.com store does have the Thetford 550P MSD and the Dometic/SeaLand 975MSD Sanipottie...check 'em out.
