My best "guess" is that it isn't rust. On most trailers, other than the frame that holds the whole shebang off the ground, there isn't a lot of ferrous metal at an altitude that could rust so badly that it would run down an outside wall...even surface rust on the frame doesn't usually "run" down anything; it just quietly sits there and rusts.
The back frame of the refrigerator is ferrous, but, even if it started to rust, it would be surface rust for a whole lot of years and then, it still wouldn't have any "drainage" from the top vent.
The only thing that comes to mind is that maybe you have a mud dauber nest up in the vent for the refrigerator and you have condensation, or maybe a leak (hope not) that is causing the dauber tube to liquefy...it would be a reddish brown drainage that would clean off rather easily with clorox. There is a refrigerator condensate drain pipe, but, they drain out of the bottom vent; you should see a small hose poking out of the lower vent...that sometimes drains liquid, but, it is condensation from the plates inside your refrigerator and wouldn't be draining from the top vent.
The only way to find out for sure is to pop the vent cover off and take a look see...