ceiling and walls

I used Luan, 20 sheets for a 22' trailer.

See if you have a specialty plywood in your area that sells only plywood. We have one here that has an amazing stock of all sorts of plywoods you could ever imagine in all sorts of thicknesses.

Luan plywood is common in RVs due to it's light weight. The forests where it comes from are over-logged and causing environmental damage. Would be nice to use something that is sustainable.

Quicksilver uses teak and monkeypod wood. Salvaged from a t-boned ketch in San Pablo harbor. John the ship's carpenter had to steam it to conform to the curved roof. Raw brand new teak and monkeypod today would be worth $10 - 15K OUCH

I think I am failing in love with new to me single-sheet roof on Lance.
16 years of baking on the sun and still solid. PO used some kind of liquid sealer around the vents.
Looks ugly, but stays soft, so no cracks and no leaks.

Whatever you use, DO NOT use that gross fuzzy stuff like a lot of European conversions use.

As others said, keep it readily replaceable. My personal preference is for a light color. If I were doing it, I would use luan and paint it.

My choice is whatever looks good to you that you can get a Home depot or lowes for $10-20 a sheet. Reason...I had to replace one sheet of paneling after fixing a window leak. Replacing what the OEM installed cost me a fortune. Had to order from Al-right at $100.00 buck a sheet and truck freight charges added onto that (to big for UPS or Fed ex) Total price was crazy and couldn't find any RV places had any I could buy from. Read their fine print in link and you'll see why I say the cheapest.
http://www.all-rite.com/paneling/wall-paneling-4x8

I always thought that if I ever got to build and RV,I would use some beutiful real wood paneling like the good old days. We had a TC made that way and it was so rich looking..
Birch paneling is readily available prefinished and is relatively cost effective. That would be my first choice.