what about building it out of wood, framed like a house, with plywood on the inside and outside, and everything glued and screwed. Flat-roofed.
You can use wood, but I think you are better off treating it as a "stressed skin" structure with wood reinforcement, rather than stud frame construction. That means the foam is also structurally glued to both sides. Houses don't have to take the pounding and sharp jolts that a camper does... well, not unless you have an earthquake.
I made a camper similar to that 20 years ago. The plywood was 2.7mm luan inside and out, with 1x2s and 3/4" XPS for the interior of the panels. An important feature was the layer of fiberglass that encapsulated it. I was very impressed with its ability to knock tree branches out of the way.
If you are considering having a box made, check out Total Composites:
https://totalcomposites.com/ The owner is active on this forum (name Victorian). Boxes run around $20k or so... it's all the other stuff that get's expensive. They are located in BC, so from Portland you could pick it up and avoid shipping (and maybe Chinese import taxes; that's where the panels are made). It looks like they use fiberglass tubing in the panels if you need structural reinforcement. Nice thing is you should be able to mount stuff on the outside and not worry about sealing holes to prevent wood rot.
Even though you said you only need it to last 5 years, you are going to sink a lot of time and money into making this the way you want. That's true regardless of the materials you use. I think you'll be happier 5 years from now if your rig is in great condition and ready for more adventures, rather than ready to fall apart...