We're getting a couple terms mixed up here. A porta potty is a self contained unit. The bottom of the potty is the "holding tank" and can easily (and cleanly) be removed from the boat and simply dumped into any toilet. It's actually a very simple and trustworthy process. The standard size is about 2.5 gallons, while the larger capacity one is 5 gallons. Even 2.5 gallons is actually quite large, especially for just a few people for the day or even overnight.
A porta potty can also be had in an "MSD" version (Marine Sanitation Device). In this case, it has fittings on the back side that can be plumbed to hoses - those hoses can go to a pump-out fitting on the topsides, or to a macerator, or a combination of the two. You can even plumb a porta-potty to a separate holding tank with a transfer pump.
Now, with that being said, I'll add that my favorite setup is a VacuFlush - but that requires A LOT more work to be installed, along with a couple thousand dollars.