I have a 99' Fleetwood Wilderness with a single rooftop a/c unit. I just purchased this camper 2 weeks ago (as-is). It's in great shape and we are very happy with it. To my question and troubleshoot...
The first thing I would look at is the baffle or divider that is above the interior grill. It divides the intake air from the cool side. The baffle in mine was out of place and all the tape was falling off. I would also remove the vents and look at the seal tape. If it has loosened and pulled away, the cool air will be leaking into the roof cavity rather than all of it being forced into the living area.