I think your best choice is likely to be the Raritan "Marine Elegance." Read all about it here:
Raritan Elegance toilet I recommend that you include the optional "Smart Flush" panel that offers 4 flush options. Easy to explain to your guests which button to push when.
It's designed to use draw flush water from the onboard pressurized fresh water system..that you have an additional 35 gallons to serve the heads makes that ok.
Now we need to address the rest of the system...which, because almost all of MA and most of the rest of New Engliand waters are "no discharge," means you're limited to a holding tank.
I'll be glad to help you spec out a system that you can maintain easily if you'd like to give me a shout via email.
