With that wall thickness you can easily create a thermal break by staggering studs/framing.
Just replace the 2x4's with 1x2's or 2x2's, and the insualted with rigid XPS foam.
For ceiling, you could do a truss'ed design to maximize the thermal break...
Many RV companies already do this, but with a radius/arch. You could easily make them flat
Like this, but done horizontal.
I considered these early on, but my camper just isn't large enough to justify it.
Such a thick wall really soaks up interior volume, when you already dot have much!
With a large camper, I feel the benefits are there....
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