The only way to tell how the roof is constructed is to open it up. I had a seam in the ceiling that leaked so I opened it enough to see a little and get a camera into. The ceiling joists are 2x2s 16 inches apart. The only thing on top of them is the sheet metal roof. Under them 1/8 inch wood sheeting. The leak seems to be caused by a drooping or broken 2x2 running side to side with a vent in the middle attached to it. This causes a pool of water beside the vent. The temporary fix is strips of wood running across three of the 2x2 joists to elevate the low spot. I know I must remove a ceiling sheet and deal with the bad joist. I wouldn't be surprised if I broke the 2x2 by stepping on it. Now I put planks across from one side to the other to support my weight when on the roof. And wonder if the walls are strong enough.
I hear good quality new RVs have steel or aluminum joists. Maybe something like metal studs?
A friend of mine was so disgusted with RV construction that he bought a used school bus, took off the roof and walls and built new ones with heavy steel beams. Very strong. Very heavy.