Bill-
First, welcome to iboats!
I don't have a 33 Mariner but my 34 Silverton had a similar problem.
Mine originally came with an oversized portapottie (6 gallon).
The previous owner opened the bilge compartments and measured the largest space. He found a similar-sized poly tank (20 gallon) and installed it below the floor. He had to cut some of the floor framing but it was all replaced and reinforced.
He installed a 3-way valve for the black water (tank-mascerator-pumpout), rigged up a new SeaEra electric head, then ran all the lines and wiring.
The end result is very nice. Pumpouts are easy and the tank has enough capacity for 4 people for a week. The electric head has been maintenance-free for the past 5 years and requires NO pumping- just push a button!