[Festus2]

The ceiling should be "tight" to the roof joists and there should be no sagging or give along the joist/rafter lines. The luan ceiling should be securely fastened along these lines. There will be some give to the ceiling in the area between the joists/rafters but the give or flex should be the same all across the width of the ceiling.

One section or area should not sag or have a significantly more "give" than any other so in that sense any sag would not be normal.

You can push up on various areas around where you feel there is sagging and any upward movement should be uniform between the joists.

If you feel or can see that there is sagging, you should contact your dealership and/or Keystone to make sure they have this problem being reported and on file before your warranty expires. Waiting until after the expiration of the warranty won't give you much satisfaction.