It is "never" necessary to remove any headliner, sheetmetal or for that matter, any other interior component, to replace any segment or quarter panel.
The secret is kowing how, and the proper use of the proper sealers and olympic rivets.
To replace that particular segment costs less than $1000.00.
The total labor involved is less than 6 man hours. Materials, 110 olympic rivets, 1 tube of vulkem sealer, and the segment. The segment should be plasticoated, in spite of the fact that it comes coated, if for no other reason, than to coat the rivets.
How do I know.
I wrote the "Crash book" for Caravanner Insurance Company, the old insurance division of Airstream, in 1970.