stick and tin RV's have some pitch to their roofs, side to side - not a lot - and a rubber membrane over that. Some say the rubber isn't good for more than 10 years, others think less than that.
So you'll want to probably go metal or maybe fiberglas.
As for structure..RV's have little trusses from what I've seen i build vids and cutaways - 2x2 framing with an arch on top.
Easy enough to 'scab' something likethat on top of the roof and sheath in the sides with metal and then roof over the top after some plywood sheathing.
Advantages I see is you don't have to do anything to the stock interior ceiling at all.
You can add as much insulation as you want.
Make the shape anything you want (more square, more round, flat, etc)
The new roof could cover the roof exits so you'd have no leaks.
room to run anything, wiring, plumbing, heating and a/c ductwork, etc.
Without putting the insulation inside the ceiling height woulnd't change..that's good!
Hmm..now you got me thinking. Even if you did it it steel it might not be bad, cost and time wise.