The way our tri-fold sofa works, I don't think there would be enough room under the seats for the holding tank to swap the jack-knife sofa out for a tri-fold.
If you are connected to city water all the time, you could just remove the fresh water tank and cap the lines. I did this on a small popup we owned when i valued the storage space more than a fresh water tank. I reinstalled it when I sold it. Could keep a few gallons of water on board for emergencies.
2015 Coachmen Freedom Express 292BHDS Blue Ox WDH
2015 RAM 1500 4X2 CrewCab, 5'7'' bed, 3.93 gears, 5.7L 8-speed VVT, Payload 1608, Max tow 10,400