I built our truck 14 years ago using FRP sandwich panel on a light weight steel frame.
In hindsight, most of the steel was not required.
I used 2 different panels, a honeycomb core and a closed cell polyurethane foam core. The honeycomb was a mistake. Its insulation properties are very poor and urethane is almost twice as good as polystyrene of the same thickness.
The best panel came from this manufacturer in Queensland Australia. There is a bit of info on their web site.
http://vanglass.homestead.com/index.html
The build strength that can be achieved with this material (all glued together with Sikaflex 252 and 11FC) is astounding. In 14 years of serious bush we have NEVER had a failure of a glue joint.
There is also a bit of info in the link in the sig line below. 14 years and counting.
I had a friend build this camper on a Defender (yes, it is a Defender under the skin) about 10years ago.. He made his own sandwich panel (FRP/urethane) by vacuum bagging it on his shed floor. There is NO steel in this build.
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Light weight, exceptional strength, excellent thermal insulation, good looks.
Cheers,
Peter