The design in my mind is as follows:
4 X 8 HF trailer (got that).
Floor: 3/4' ply with 2 X 2 frame - 5' X 8'. Butt-joint in middle along a 2 X 2 frame member. No insulation. Tar coated. Cut out for wheels.
Sides: Sandwich 1/2' ply, 3/4' foam, inexpensive paneling (1/8"?) with 1 X 4 vertical stringers to frame the door, bottom where it attaches to floor and where inside shelf and bunk hang. No galley.
Roof: interior paneling, 2 X 2 stingers, 1 1/2 foam insulation, 1/4" luan on exterior. All installed from the outside using the ingenous method I saw on this forum where the sides are finished inside and out with required spacing left for inside paneling, stringers, foam and outside layer.
Outside corners radiused with a router and epoxied. Painted with latex primer and color.
One door, 2 windows, roof vent. A few 12V electric lights and fans.
Thought about covering the trailer with 3/4" treated plywood to allow camper to be removed and used as a hauling trailer.
Drawings forthcoming. Go ahead boys, let 'er rip.