Re: Back for more - portapotty vs. manual head
A typical porta potti has a holding tank that is much less than 6 gal. Two adults on a long weekend cruise would fill it, so your endurance would suffer if you switch over to the porta potti.
Since you can at least remove the porta potti to dump it in a toilet at the marina, you might consider whether that is more convenient than pumping out your existing holding tank, which means moving your whole boat.
You didn't indicate your sailing venue, but if there is a possibility of sailing past the 3 mile limit (say on your way to Block Island) you might dump your existing holding tank overboard if you have a way to do this. Otherwise, there are a number of options for pump outs along the CT and RI coast. Block Island and Westerly (Watch Hill/Napatree) have free pump out boats, as does Stonington and Groton (e.g, Mystic River).
My only reservation is the endurance you'd have with the limited capacity of a porta potti. We have a power boat with a porta potti and you need to plan for it filling up. Our sailboat has a 23 gal holding tank and we have to plan for that, too, but obviously with more latitude.