One thing I wonder about is the insulation and R-Value performance of the Lemker panels. XPS (TC) has higher insulation value than PET (Lemker) foam.
I never heard of TC using XPS. They used PU in the beginning, pretty certain. At any rate, properties of these structural plastic foams isn't very different for the same densities, and
R value is between 4-6/inch. The light ones are typically 4-5 lb/cu ft density, compared to common XPS foams that are in the 1.5-2.0 lb/cu ft range. For that higher density you get much higher strength values, 5-10x typically.
R value changes little with density, because the main heat transfer path is between closed cells of air, not the plastic.
I chose PVC because it was stronger by weight than PET. But PET has better temperature resistance as I recall.
Regarding your concern about insulation, 1.25" of foam with no thermal shorts would be plenty for almost everyone, IMO. The optimal amount of insulation for a camper is a lot less than it is for a house, because extra thickness takes up valuable space, and you need a decent amount of air infiltration for your occupants regardless. If your plan is to camp in extreme winter regularly though, you may notice a difference.
I got this from Lemker's first post on ExPo last year.
https://forum.expeditionportal.com/threads/lemker-overland-introduction.245869/
