Standing on the roo is not a problem, BUT you need to keep your weight off the end caps and on the rivet lines. The rivet lines mark the ribs. These will support your weight. If you want to spread the stress on the roof out you could place a small sheet of plywood on the roof bridging multiple ribs and give you a larger workspace.
Replacing a shroud is normally a 4 bolt/screw job and may be able to be done whitout getting on the roof. It depends on your ladder, and how long your arms are though.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato