A marine toilet and holding tank will require adding at least one below-waterline thru-hull for the flush water inlet line, several feet of hose from the toilet to the tank and at least a 15-18 gal tank to have one that will last two people longer than 24 hours. All that will eat up at least 1/3 of your storage space, cost you several hundred $$ and require a lot of maintenance. There's a better way:
Replace the portable portapotty with an "MSD" portapotty. 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, vent line and a deck pumpout fitting 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