Wow, that's quite a plan. More work than I could ever consider. I like the idea of plopping a box on that's all ready to build in the living quarters. Holding tanks I know a bit about. My truck conversion has 2 100 gallon fresh water tanks. That's enough to last a LONG time. It's got 75 gallon gray and 25 gallon black. That seems to work plenty fine for the two of us for a week on the road. Obviously we still have 100 gallons of fresh water left at our first needed dump. You are planning on putting a cut through from the cab into the living quarters, aren't you? Good luck with this project. I'm sure others will chime in.
'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio