You can (and I do daily) walk, or at least move around on the roof. Stay on the rivet lined, that's where the ribs are, with their support. Access the roof through a ladder propped against the awning, or if you don't have an awning, tape a pair of old towels around the vertical area of the ladder where it would touch the sides of the trailer, and lean it against the trailer. That way, the ladder won't scratch the trailer. And like stated above, stay off the end segments.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.