I was thinking the same thing on FRP walls. That is the only way I can figure everything just came apart clean at the floor frame level like that. Any sort of superstructure framing would show some evidence remaining above the floor level. And the trailer structure stayed largely intact with a traditional steel frame walls and ceiling, while the truck simple came apart at the seams. Could have been a homebuilt as well, except we all seem to err on the side of overbuilt. I did see a youtube video of a home build of a 5th wheel trailer than had not one bit of metal beyond the chassis, the entire superstructure was plywood. My bet is on FRP though.
Good point on the cab cutout. I was planning a large cutout, and my only thought was how to make the hole to work well with a seal/boot. Now I'm thinking about how to reinforce the opening to make up for the lost strength. Should have been obvious in retrospect. On the other hand, would even an unmodified cab be expected to hold up with 40,000# of truck rolling over on it?
Any guess on how long that entire rig was? looks like a 30-32' (plus tongue) triaxle trailer, and 40-45' truck. Was it even legal? Was this one of those over length race rigs that we all wonder how they even go down the road without getting stopped? Could be part of the cause, those really long rigs get in trouble fast if you get just a little bit out of shape, like crosswinds etc.
Regardless, terrible tragedy for the family and our hearts go out.
