My experience on restoring old campers is to redo and take out all the rot,not to just patch. I would compare patching the wood instead of replacing to having a car, not pulling dents out and then filling with bondo. Is it fixed,yes but eventually it will show back up. If yours has the aluminum trim on the outside that many of the old campers have thats probably the culprit. What happens is they put a putty between the aluminum skin and the aluminum trim. This putty gets old,dries out and cracks. It funnels the water in. So you fix the rot today them months down the road its back. You may have two issues,rot and the source of the leak. It soulds like a lot of work and it is but its not as bad as you think. And most campers are easy to figure out how they are made. If you intend on keeping it fix it right. I bought a camper once that was "patched". If it had been done right it would have been in good shape. I wound up having to replace about half the wood in the camper. Another camper I had to replace all the wood structure.
I have a couple of my campers on line with rebuilding photos
The Aristocrat at
http://good-times.webshots.com/album/560257227sGqfjB
and the scotty
http://outdoors.webshots.com/album/560245562OMCsZD
A good site to go to for answers is a vintage camper site at
http://repairingyesterdaystrailers.yuku.com/directory