We had a similar challenge in our home built cargo trailer conversion. Saw a picture of another guys way of handling it, and it made the job easier. We made flat panels out of luan, then covered the joints with strips. My joints were at 4', but I did cover strips at 2' just to look uniform. Since I had metal tubing to deal with in the roof, I installed furring strips North-South and on each cross member, to mount the luan panels to. Call me cheap, but the best purchase I ever made on this project was a pneumatic stapler. Already had the air compressor, and once I got the settings right, it made the job a lot easier.
A picture probably will explain the idea better. (apologies for the forum watermarks, if they appear...)
Ceiling close-up with our homemade cabinet.
Finished view.
Looking at some interior pics of your era Road Runner, the factory took a similar approach, what I call "cutting the corner". They had maybe 2-3" wide, East-West strips of wood, each piece mounted top to bottom of the curve, making the transition from roof to wall. Appears they also put in cabinets, which they could easily radius the back, and the interior rear surface wouldn't matter so much.
Here was the inspiration:
Teardrops n Tiny Travel Trailers • View topic - A 7X12
Edit: Here's another fellow building a roadrunner, good original interior pics. (it is from that other rv site however...)
http://www.rv.net/forum/index.cfm/fu...d/29289176.cfm
Those pics reminded me of the local guys that built the skateboard ramps. They literally wet the plywood with a hose repeatedly until they got the curve they wanted, then finished mounting it to the support bracing so it wouldn't lift. I've not done it but it would be an interesting experiment.
Good Luck, show us the finished product.