For structural sheathing we use standard a/c plywood (never never any pressborad or particle board). For our interiors however we use either Birch veneercore plywood, Baltic Birch or Apple ply maple or birch plywood. We like to use 1/4' birch ply on the finished walls and ceilings because it paints just like the sheetrock with the proper preparation and will therefore be easier to repair if damaged later. I never did like the printed platic luan that most manufacturers seem to use. Ad far as $$$ go it is indeed more expensive but in the great scheme of things a very amall percentage of the overall cost. Good materials are easier to work with bringing labor costs down. On our latest coach we are using a real cork floor and it not only looks beautiful but is warm and soft to the touch.
Tony
ToolSet Motorhomes