Just purchased a used 1988 Charmac two-horse.  Used, but covered and taken care of.  Rust is very minimual, just surface.

The worst of it is the ceiling.  There is loose paint and some peeling, with surface rust spots here and there.  The rust comes off easily with steel wool. 

Strangely, the areas where the paint is OK, there are "shadowy" gray spots that look a little like mildew.  The spots missing paint have the surface rust and when I scrub them on the outer edges, I notice a "gummy" coat of something under the thin coat of paint.  The "gummy" stuff is black. 

Is this perpetually damp paint/primer that needs to dry out?  Did the previous owners re-paint the ceiling and use the wrong primer? 

I'm going to take some milew remover or bleach to see if the "shadowy" spots come off.  If they do, I will assume they are mildew spots.  But, why would it be mildewy?  Condensation?

I will find out if there are leaks today.  It has been dry out and last night and today it's been pretty rainy, so I will look for wet areas in the trailer and see if the ceiling has a lot of condensations.

Thanks!

That is oxidation of the paint.  Soon, the oxidation will be of the metal and that is rust.  This is my understanding.

There are several auto sites that give instruction on how to handle oxidation.  Here's one link, but it is one of many:

http://www.properautocare.com/resduloxpain.html