Forgot to mention on my comments of ladder useage wen getting to top of  trailer. I recently removed some inside paneling on the '56 I"m  rebuilding and noted a interior structure system I was somewhat surprised  at. When you look at rivet lines on outside of trailer you tend to  think the skin is riveted to a series of structural ribs runing vertically and horizontally. Not exactly the case. I learned that the major structural ribs are mostly vertical and have rivets spaced about  an inch or less apart. You can note other rivet lines with rivets about 2 inches apart. These wide rivet patterns go into both vertical and horizontal bracing pieces that do not connect to an solid structual member. The serve as bridging across an expanse of aluminum skin just to give it some rigidety and not permit the skin to bend or flex to much. I found the inside skin is not riveted to these braces and have seen the inside skin with bracing of its own, at least around the vent area. You may have wondered, as I have in the past, why the outside side panels seem overly flexible when you push against them when they appear
to riveted to a brace. There is only bridging behind the panel.

The point of all this is to remind one to be careful when placing a ladder against or standing on a wide rivet line as there is not much strength there.

Roy