The correction for a coach with a dished roof around the air condition that Keith is probably referring to in the previous post is to remove the air conditioner and insert thin plywood into the space directly below the fiberglass by using the A/C cutout opening as the entry point. You keep the plywood very even under the gasket matting surface, and use the minimum thickness to correct the dip. This correction does work, but it is a lot of work, can be expensive, and should only be used when necessary.
Gerald