When I replaced my roof fan I was able to put my full
weight on the lower part on the kitchen side of the roof by staying close to the edge and using a piece of foam board to distribute the
weight. I would never get on the center section of the roof even if it was supported inside. If I were doing what you are, I'd either move the trailer or perhaps rent some staging. Cheaper than repairing a caved roof. Good luck. Raz