I wouldn't do it because, in addition to the toilet (and btw, they all require a lot of maintenance)--you'd have to install a thru-hull and seacock to supply flush water and a holding tank and related plumbing that would occupy at least 25% of your storage space. On any boat smaller than about 28', an "MSD" 5.5 gallon portapotty is a much better solution IMO. 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. 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.
If you trailer to the coast and regularly get out beyond the "3 mile limit" (where you have to be a dump a tank legally), you could add a y-valve and overboard discharge pump to the pumpout line. However, that would prob'ly mean installing a below waterline thru-hull and seacock, so may not be worth the effort.
Check out the Thetford 550P MSD and the Dometic/SeaLand 565MSD, both of which are available from the sbo.com online store.
