You start at the bottom and work you way up. The top of the lower panel slides into a channel at the bottom of the next upper panel. If you work carefully, you can reuse the wall panels.
Is there no access from the inside to see what is going on?
Again, read some of the Truck Camper University right here
https://forums.trailerlife.com/index.cfm/fuseaction/thread/tid/25966285.cfm - scroll down to "Camper Structual: Repairs"
Don't be scared by what you see there, though.
This thread
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/18228847.cfm shows a bit of the wall designs.
