You might find this link interesting:
https://www.doityourselfrv.com/ever-wondered-171000-diy-motorhome-looks-like-speechless/
From the link:
"Wood/Epoxy with fiberglass (or carbon fiber) is significantly stronger than tubular aluminum which has a skeleton, and then a separate skin. This truck is built like a multi-hull sailboat, structure and skin are one, making it very strong.
I’ve been in a Gale sailing a 44 foot sailboats built like Shachagra and it took a pounding that no truck is ever going to give it, made me very confident in that method of construction."
There is a build thread:
Great threads here, great to see lots of projects. We just finished touring Europe and Turkey in our home built on a International 7500 chassis. Large, 33K, 400 gallons water, 200 gal diesel, all diesel fired (stove, heating) with a 2K W solar array and 2100 AH of Lifeline batteries. The trip...
expeditionportal.com
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You might also look at the following wood+epoxy camper by
@IdaSHO
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@Teardropper has a
book on building teardrop trailers which might be useful to you even building a truck camper.
A long-term project that I'm quite pleased with. I have watched so many teardrop builders struggle (which is probably just part of the process). I hope this takes some of the angst out of creating one of these remarkable campers. 192 pages, 221 photos and 31 pencil biting chapters...
expeditionportal.com
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Although larger than a pickup truck camper, you might find some information in the following two threads:
I recently purchased a 2007 Oshkosh MTVR. We intend to convert it into an overland expedition type vehicle and do full-time RV living. For those not familiar with the MTVR, it is an all-terrain military 6x6 truck built for the U.S. Marine Corps, and it is...big- nearly 12 feet tall. MTVR...
expeditionportal.com
I know that most people are trying to build something light enough to work with their particular vehicle, but our MTVR has a 30,000-pound payload capacity, so there may be solutions that wouldn't work for other people, but would work just fine for us. Our MTVR's flatbed is rigid, and it's...
expeditionportal.com