The roof will surely be strong enough for an adult, up to say 225 lbs or so. More if cautious. Modern rubber, TPO and fiberglass roofs all have a luan plywood backer (substrate) under the outer skin.
If your weight is at the upper end, just watch for signs that the roof is flexing noticeably. If so, cut some 12"x12" 1/4" plywood squares to act as snowshoe-like stepping pads to help spread the weight. Move them around and stand/kneel on those instead of directly on the roof surface.
Installing a ladder is often not a simple thing - you have to find, or add, backing material behind the end cap and roof to provide a place to screw or bolt the ladder mounts. And those ladders aren't all that useful anyway, difficult to climb straight up and then clamber over the edge. I would suggest either a tall step ladder or extension ladder. If you need to take it with you, a collapsible (telescoping) ladder works well and can be used anywhere around the RV.
http://thediyoutlet.com/products/16-2ft-a-shape-aluminum-telescopic-extension-ladder?gclid=Cj0KEQjwl_6oBRDHxNGz6ueJufMBEiQAvm_k_gTzWG7gAkwA4IO_D_S2oLh7HTURAlo0vozy0-1VZqwaAq798P8HAQ