I wouldn’t start with that vehicle, I’d start with a something like a van front motorhome that had camper damage, like maybe a tree fell on it. They can be purchased very cheaply, will have a lower floor, and much of the appliances and cabinetry, etc. can be salvaged.
I have built a few. I start with a roll cage, then build the body using the foamy method. It’s fast, cheap, easy, and extremely strong, durable, rot proof, and light weight.
I use a double layer of 1" foam panels, staggered for added waterproofing properties. Rounded corners can be achieved using a hot wire knife or a router, if you want them, but square corners are popular not too.
I avoid any holes in the roof like the plague. I vent everything to the sides using rain proof louvers. Solar brackets, or cargo racks mount to the sides, zero roof holes. Done right, it will never leak, and will outlast the rest of the vehicle.
The 2" foam floor/walls/roof offer superior light weight insulation as well being extremely strong and durable. I coat the with smooth rubberized truck bed liner both inside & out, before adding finishing paint, or possibly interior paneling. I DO NOT use wood paneling.
I even make the cabinets out of foam, because it’s lighter, stronger, and more durable & moisture proof than wood. Hidden, foolproof, simple, lift to open wedge type latches for drawers. Sliding cabinet doors instead of swinging ones, sliding or accordian doors for rooms.