Sounds like it's going to be a cool camper. I think you probably would only need one roof air for living quarters that size. My previous class A rv had one roof air and the generator was a 6k which seemed to work just fine. My present rig has two roof airs and it's got a 8k generator that seems to be plenty of power. I don't know what the smallest diesel generator you can get. My truck has 25 gallon black and 75 gallon gray tanks. My wife and I can go several (I'd guess 5-7 days) before filling up the tanks. And that's taking lots of showers because we cycle and run a lot. Usually you only need half to a third of the black tank vs. gray tank.
Good luck with the build and post lots of pictures and questions. There are lots of people around here that have built their campers and know lots of stuff.
'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