The walls are thin, so a lot of weight saved there.
The floor is a laminated sandwich of very thin plywood on both sides of the aluminum/Styrofoam core. But all of the lightweights are using this system now. There is no real plywood in the floor.
As far as I know, there are no rafters in the ceiling, just another laminated sandwich, but with no framing like the floor.
The axles are the bare minimum for the camper weight, but again seems to be standard procedure.
The main frame is made up of three pieces of sheet metal welded together, and not a real I-beam like on older trailers. It is very light, and probably not very strong.
But does all this compromise the structural integrity? I haven't seen any evidence of it on ours, yet. But it sure does creak and groan when we drop it on the truck, as the weight is being transferred. I wonder if some day something is going to give out in that fifth wheel area. It is flexing, no doubt about that.
But if you want a lightweight, they are very nice campers. And overall, we are happy with ours.