Roof-How on earth do you safely sit up there.

I really dont know how others have done it, but in my head I am envisioning something long enough that it reaches all the way across the trailer so the weight is distributed onto the walls, and solid enough it doesn't sag in the middle and dent your roof. maybe two 2x4's about 4 feet long running parallel with the walls with some 3/4 inch plywood between them. so your weight is distributed onto the walls.

Invest in a couple ten-foot 2X6's, to lay across the span of the roof, and maybe a couple 2X4's that can be cut to two-ft lengths. Tying the spans together will help spread the pressure out. One should never ever put their elbow on the aluminum, let alone their whole body weight. I have walked on a roof of a 36' fifthwheel before, being careful to step on the rafters...but I would not do it again nor suggest anyone else try. My Dad always told me the biggest problem that any roof could have is someone walking on it.

Funny, at the dealer I worked at we would always be on top of the roof for annual roof inspections. There are even ladders mounted to most of the rigs on the back to gain access.
There has only been a hand full I've worked on where I haven't been on the roof.
Usually not on a Shasta's 1" thick roof construction with rot though!

JLR…. If you are concerned about weight, even when spanning from wall to wall with a 2X10 (there will still be some flexing with that), then you could create a makeshift scaffold with 2 ladders on either side of the trailer and let the 2X10 bear on those. Then keep moving the rig down the trailer as you do work in different places. Of course the ladders need to be taller than the trailer roof ;-)  You could also put the 2X10 on blocks, spaced to the width of your trailer, so the blocks would rest on the walls and there would be no bearing on the roof at all from flexure. You're not being paranoid; you definitely don't want to walk or sit on the roofs of these old trailers.