Welcome to iRV2 and the vintage Newmar owners club .
I'd have to have a real good look at my ceiling panels , but I believe they go across the top of the wall ; so removing them maybe problematic.
Ceiling panels also continuous over all interior walls.
A while back a member posted pictures of his roof replacement , and the pictures he posted may help .
Maybe some of the info you require is in the " Sticky " posts at the top of the list in the Newmar owners forums .
"Quick tips and easy mods ", have links to just about every repair members have undertaken on their Newmars