I do all of the electrical work for one of the biggest caravan manufacturers in NZ. Their method is a box section chassis, plywood floor on that. Then they have wooden framing for walls and roof. Exterior cladding is a sandwich panel, the outsides are aluminium (painted white), and the inside is PVC. The whole thing is about 5mm thick and comes on a huge roll. The inside cladding is thin MDF with a veneer on top.
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