Welcome to the forum.
You will need 3 or 4 of the white-faced luan sheets to replace your ceiling, the originals were 40" wide instead of 48". You will need to cut them lengthwise as well, as the trailer isn't 8' wide. When you get them cut the right length and width, you will need to glue them to the roof. I have had the best luck with LocTite foam and panel adhesive. It comes in tubes that you put in a caulking gun to apply. Use it like contact adhesive, put it on, stick the two pieces together, wait a little bit, and stick them back together. I use a brace made from 4 sections of 2x4's set upright and a piece of plywood across the top of them. Lower the roof down onto this after you put the ceiling up to hold it in place until it dries.
After you get the ceiling panels installed, you will have the joy of figuring out where all the curtain tracks and other hardware goes...
Two wrongs don't make a right, but three rights make a left.