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You could use a regular ladder. Then cut 2 or 3 pieces of 1/2" plywood 2' W X 3'L and lay them across the roof as you walk moving them as you walk. This distributes your weight over a larger area.
Jayco Jayfeather uses 3/8" plywood on top of conventional wood rafters, so there's no need to use additional plywood sheets to spread the load. Trailers with unsupported aluminum or some vacubonded roofs are a different story.
As long as you weigh less than about 300 lbs, prop a ladder against the side and climb up there. Just make sure you don't have any trapped gravel in your shoe soles that may scratch the EDPM.