The good news is those old trailers had real, but very thin, plywood ceilings and then contact paper glued to that. The water damage might not be as bad as you think maybe the paper just let go. Is that apartment electric fridge or a old gas electric? You might want to check out and see of the old furnace works. They were great when they worked because they required no electricity but, it can be hard to find parts for them anymore. It's always a good idea to add a Carbon Monoxide detector to any camper.
Thanks for the pictures. I love seeing pictures of any camper especially the older ones.