I posted
this for someone asking about ribs. This is a 74 motorhome, but probably very similar. Between the ribs was fiberglass batt insulation and a lot of empty space.
I am going to put paneling on my walls. First try I put a piece against the floor and wall, tried to push it into the curve and it broke. So I made a buck that I can clamp the sheet to on what will be the bottom and slowly pull it down using 2x4's and pipe clamps. Have done 2 scrap sheets this way and neither broke. I am now debating if it would be better to cut the window opening before clamping or after. Before I see the weakness of the opening in the panel might affect it, after it would be a pain to make the opening.
There is no nice 16-32-48 spacing on the ribs, you can see from the rivet lines outside. An 8 ft sheet does go from my floor to the aluminum trim that runs front to rear.
I also put foam weatherstrip behind a piece of 1/8x2 aluminum strip over each rib. This hopefully will be a thermal break and give something a little wider to mount on.
John