You don't need temp control for epoxy, it just needs to be warm enough to cure. I used the slow cure for everything even in winter (avg daily temp range low 20s to high 40s) and just threw a plastic sheet and cheap electric blankets and comforters over it once it got sorta solid. The blanket raised the temp ~50F over ambient.
It is best to to get all the layers done in one go, so you don't have to sand in between... but you can also use peel ply. But... if you want a really nice smooth finish, this is hard to do with hand layup, at least for me. I don't think it's worth the bother unless you want curves and such, and you won't have that with an exoskeleton.
You can buy panels already made from Carbon Core, with the PP honeycomb core.
https://www.carbon-core.com/product/composite-panels-sheets/ Price is quite reasonable. The downside there is poor insulation compared to foam. The Everlander build (
https://everlanders.com/build/) used these or something similar.
If you want to build your own panels, I'd suggest PVC foam from Carbon Core. It's much stronger and glues better than XPS. When I bought from them, freight shipping was a flat ~$350, or free if you ordered enough ($2-3k in one order?)
https://www.carbon-core.com/product/pvc-structural-foam-sheets/
Check out this place for the FRP sheets.
https://www.vetroresina.com/en/product-category/g-force-j-force/ The US plant is in Greenville SC, and I know of a couple people who have used them. They have options that are very robust.
Do you have any family members who can help move things around?
Also, what year is your truck? Does it have a boxed (stiff) frame? Payload?
I wouldn't be shy about making your cabover as long as your cab. Make it so you can sleep fore-aft, and have lots of room.
I think you can get the shell done and mounted in 6 months if you are focused and diligent... not lazy like me!