Hi Jim,
I believe they use luan only on the interior side of the wall. The exterior is a thick sheet of fiberglass right up against the aluminum studs with block foam insulation filling the gap between studs. But please confirm this with the factory. If you talk with them, please post your findings here for the rest of us.
One other thing Phoenix does with their wall and roof panels is aluminum-frame every window, roof vent, and a/c unit opening for a more solid structure. Doing so also avoids distortion around them.
We have owned our full-body-paint PC-2350 for over 11 years, still with a very nice shine and reflection today. Our exterior walls show minimal wave, the same amount it had when brand new. All motor homes are not 100% perfect and ours is no exception. But it's dang close to perfect.