Hmm.... I hadn't looked behind the c-channel as yet. I thought that was the outer skin, but, after seeing the rust in the picture, I see that it's certainly not aluminum!
I do know we have a lot of steel repairs we need to do to the rear end of the frame. Rust, missing pieces here and there, etc. Nothing will surprise me too much once I remove the belly skin and floor and get a really good look at the condition of everything hidden.
Aerowood, I do like your ideas on how to repair the c-channel. Sounds easier than replacing the entire thing, provided that the rest of the channel is structurally sound. Starting this weekend, I'll be removing the belly skin, and then I'll start having a better idea of what's what underneath.