Here's a couple posts from my experience so far and how I approached it:
After roof raise, primed and sealed but no insulation:
Bought a whole buncha foam (rigid polystyrene)
Installed a whole buncha foam, of which the job seemed endless.
Finished foam with another layer of foam, so silver mylar stuff faces inward and outwards.
Skinned ceiling with the HDPE plastic sheeting stuff. It says right on the label not to use in RV applications. ;)
I have since discovered that it is sensitive to expansion so it can bulge slightly between the fastener points used. (I used self tapping or plastic rivets regularly spaced)
The material is functional, easy to clean, chemically resistant and waterproof.
I haven't gotten much further in the finishing department because we have impromptu started living in it while we sold our house and searched for another one. I'll soon start sheeting the vertical interior sides of the vehicle, and get to work on the partitions and inner walls.
plastex 48"x96" sheets, $20 each at lowes (and probably other places)
Shop Parkland Plastics Plas-Tex 48-in x 8-ft Embossed Plastic Wainscoting Wall Panel at Lowes.com
I plan on using polycarbonate twinwall panels (those plastic greenhouse walls) with this same paneling glued to either side for the interior walls. The only place there will be a lot of wood in this vehicle is the subfloor plywood and support stringers, which by necessity (for thermal break) and price (because expensive aerospace materials are not in the budget) were wood.