[Motor31]

No certain ideas of where to look. It sounds like there was some chafing of wires in the 12 volt loom near the fuse box. I say there as that is the only place I see wires from those items getting in close contact. I'd guess that you have a direct short in those two wires from rubbing. You are lucky is was in a spot after the fuse box or you would likely have had a fire.

If you can get into the area of the fuse box from behind it, look for wires in contact. You might have to remove the coat closet to look at the wires ascending from the fuse box before they split to different sides of the trailer.