I weigh about 320 and had to get on the roof of our old RV, a 2006 Class A. I did so, hesitantly, but I did not walk on it - I laid down and crawled across it on all fours or scooted. You'll get filthy, but it spreads the load out enough you don't need to worry about it. If you need to work up there a lot, I would drag a sheet of plywood up and lay on it to further spread the weight out. So far as the ladder goes, ?i personally would not trust it. The rating U.S. based on what the ladder will hold but has nothing to do with the #8 wood screws into the 1/2" plywood blocking that it's most likely mounted to the RV with.
2015 Legacy SR340 360RB
2014 Jeep Wrangler Unlimited Toad
Gigi, Poppy and Sadie Lady...On the road, but not full time!