To the OP, you might be able to find online the series of videos that Mark Polk ( of RV 101 fame ) did on the restoration of an old Yellowstone trailer. He tore it all the way down then built it back up.
I understand from what you are saying you're not planning on taking it all the way down. But watching the vids on it will show you how these things are constructed. And then as you get further along in the last vids, much of what he did as finish work will be directly applicable to your own project.
I can't see your photos so I can't really make any comments on your trailer. One thing that some folks do when rebuilding/restoring a trailer is to get it to a point where it's weather tight, and essentially a "hard sided tent", and go do a little camping with it, then continue the finish work.
