Given the choice, I'd start with a sprinter type chassis. Then you have the headroom.
But think about what it will take to make it comfortable. If you want a shower then you'll want a sink/tanks/pump, etc. Heat? If you have heat then what about a stove if you have propane? See where this is going? Gotta be pretty handy if you're going to build all this and have it be functional.
Look in the TC section, guy wanting to dump his camper. Could find a camper and gut out all the good stuff out of it. I would feel comfortable with this kind of project, but IMO, it's not worth the time and money compared to something pre engineered and constructed.
What kind of plumbing/electrical/carpentry experience do you have? Garage full of tools, power and hand tools?
Judging by your posts, this seems like it would be a huge task just to appear incognito.
You gonna black out all the windows or just sit in the dark? I don't see this being really practical. Just get a pickup and TC. Much easier IMO and stealth is overrated. I mean if a cop is worried about you living in it, you'll look like you're living in a van to the trained eye.