Unfortunately Thetford discontinued both their 550 and their 550P several years ago. The Dometic/SeaLand is the only MSD portapotty still sold in the US and I''m seeing enough complaints about it to continue recommending it.
However, if you're even slightly handy with a hole saw, it's not that hard to turn any portable portapotty into an MSD version. Note the location of the vent and pumput fittings on the Dometic tank...you can use a Uniseal and schedule 80 PVC pipe for the pumpout and vent hose fittings. I can walk you through the details.
AS for the brackets issue....Alan said,"No way would it securely hold a full porta potty in a seaway."
The solution for that: install a y-valve and an overboard discharge pump in the pumpout line and dump the tank the minute you reach waters where that's legal, reducing the stress on the brackets.
--Peggie