I once had the opportunity to see a bare frame for a fiver. I could literally push down on a section and make the hole frame wobble. I was shocked at how flimsy it was and that left a lasting impression on me but I suspect the aluminum frame that forms the walls and roof also adds to the structure, but a very week one at that.
I also see fiver's that the back in is drooping down and again is probably due to a flimsy frame that allows the structure to sag over time. I also suspect there's fiver's that have some sort of frame damage and the owners don't even know it.