I make a scaffold
out of two extension ladders and an 8' 2x10. You can see that there is a flat piece of 1/2" plywood under the ladder where it contacts the shell, with some 3/4" foam under the plywood. There really isn't much pressure on the shell, so this is really just a scratch prevention device. What you can't see is the rope that goes under the Airstream that ties the bottoms of the ladders together, and I also have a rope over the top. This is overkill, except when you're moving something heavy like an air conditioner, where it is essential.
You can stand on the roof once you're up there, as long as it's on a rib or stringer. Never-the-less, I always try to sit on the 2x10 when I'm working up there.
When I'm on an extended trip I carry a 6' stepladder in the truck so I can work on the awning and any leaking seam, but no getting on top with that.