bushpilot-
I don't know how athletic you are, but I have seen some concession trailers with a series of small fold down steps running up one corner of the trailer to allow somebody to climb up and put up the signs, etc.. Just basically small footholds but they get the job done and are very small an unobtrusive.
Check out this pic:
http://www.brothersbande.com/images/daypopperv2.jpg
on the right front corner of the trailer you can see the small folded up steps alternating their way up the corner of the trailer up to the roof. When you see them on the side of a trailer it looks like a rock climbing wall, but they climb way easier than it looks. They do have to mount on a corner though, so one foot can go on each side, I think if they were straight up a wall it would be about impossible to climb.