Plumbing the MSD pump-out was a big improvement.
If you don't know what he means by that, an "MSD" portapotty 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 can pump it out instead of having carry anything off the boat to empty it.
A 5-6 gallon model (which has the same footprint as your portable) 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.
Check out the Thetford 550P MSD and the Dometic/SeaLand 975MSD Sanipottie. Thetford has discontinued the 550PMSD (and hasn't replaced it with another MSD version), but it’s still available from most retailers…the Dometic 975MSD is still in production. They’re comparable, and not much can go wrong with either one...so if you like the Thetford, find out if sbo.com still carries it before shopping for it elsewhere. Sbo.com does have the Dometic 975MSD.
--Peggie