Ronco Plastics
Ronco Plastics marine Tanks is your best source for a tank. They make TOP quality thick-walled water and waste tanks for a very reasonable
price and have more than 400 shapes and sizes, over 100 of which are non-rectangular, and they install fittings in the sizes and locations specified by the customer when they make the tank.
As for the best place to buy
marine toilets...that depends on the type of
toilet. For the kind of use you describe I think you want one that's a no-brainer to flush and impossible to clog...and that would would be a direct drop RV toilet..just step on the pedal to open a the trap door in the bottom of the bowl and start the flush water...and everthing slides into the tank. That would require installing the tank directly below the toilet, btw. Any of the Dometic 300 series shown here
https://www.dometic.com/en-us/us/pro...tation/toilets would be a good choice.
Gray water can legally go directly
overboard, so your tank only has to hold toilet waste. The RV gravity toilets connect to the boat's fresh water
plumbing, but only use about a pint a flush...so I'm guessing that a 50-60 gallon tank, fairly shallow since it'll hang between pontoons, would be about the right size unless you plan to carry a "cast of thousands" onboard.
Plumbing is very simple: a vent line and a pumpout line.
You're welcome to give me a shout if you'd like to discuss in more detail than is practical in a forum.
--Peggie