I use a section of scaffold, a painter's/wallpaper hanger's trestle or 2x12 up top parallel to trailer, offcut of 3/4" plywood (about 24"x60") as a bridge and working platform from the trestle plank out on to the step in the trolley roof. I usually secure the plywd bridge with a C-clamp to the scaffold plank. Couple nails or drywall screw would work as well. The step in the roof is the most rigid place on a
Burro roof and the perfect place to land the end of the bridge as every thruhull including vents, fan and AC, is on the central roof facet. Knee pads are a good idea as you'll be down on them with both hands free rather than lying on the trailer to spread your
weight or draped half on/half off clinging for dear life. Can't find pix which would make this rig simpler to visualize.
You probably can install a vent cover and the little vent stack breathers and the bathroom vent from a ladder. Not going to happen with heavy, centrally-located AC. Maybe you'll never be up there again after this one job. Weigh the risks against the frequency of use and cost of gathering and storing equipment. My old scaffold frames are heavy. At 66, getting them placed and braced isn't really all that much fun but beats a trip to the emergency room. Harbour Freight has a moderate duty adjustable height scaffold with a steel plank or tray built in. About $200. Adaptable to maintenance chores around the house also and more manageable weight-wise and setup-wise.
jack