I can only speak from my experience working on my little rockwood (1996) pop up. I had the same problem with the wood panel in the rear of the camper, it was completely rotted.
It is best if you have two people to work on this project, but not necessary. Crank your camper up so that the roof is about head level. Next, look to see how the wood is attached to the exterior shell. The wood on my camper was attached with screws and some bonding adhesive. On my camper the wood was so warped and rotted that it was warping the shape of the exterior shell.First, you will need to remove any vinyl or plastic covering the wood (you may have to cut this away, but salvage it if you can) Next, take the screws out that hold the wood to the fiberglass shell. I needed a pry/scraping bar to scrape the wood away from the shell. Hopefully your camper is like mine and the support poles are attached to the side panels and not the rear and front panels (if the support poles are attached then you will need to unattach it and find a way to block up the roof while you are replacing this piece of wood) Next, I used 1/4" by 4" pine (found in the trim area at home depot or lowes) I painted mine with an exterior paint just to protect it from the elements. Next, I cut it to size and used adhesive caulk to attach it. Also, there are likely metal corner braces attaching the back panels to the side panels in the interior corners, and I replaced these with new braces and short screws (also found at home depot). Next insert a couple of short screws from the inside of the wood to the shell, (you can also use through bolts with washers and nuts to firmly attach the new piece of wood). Finally, replace the vinyl or plastic covering that should have been covering that piece of wood and adhesive caulk it into place. On my camper the plastic covering had to be completely cut away due to damage and that is why I painted the new piece of wood to keep it protected from moisture. I did this to my camper last year and it seems to be working fine. I know my instructions are not that well written buy hopefully they are of some help. Also, you can send me pictures of the project before you start it at firstname.lastname@example.org
and I can give you more detailed instructions once I see how your camper is configured.
If you are interested please visit my blog at: Camp On!