The original AC on my '73 was riveted to the roof around the entire perimeter. After removing the AC, I had a zillion holes left from the original rivets. I made a giant patch that covered the entire area out of .032 aluminum that matched my skin. My inner skins were out, so I bucked it into place, but you could use Olympic rivets as well. In my case, I was installing a new, modern AC, so I then cut a new hole in the patch for the new unit to mount in.
One thing to be very careful of, is that much of the trailer's wiring runs up the center of the ceiling. You need to be very careful with any drilling or screw insertion that you aren't damaging wires.