I'm assuming a pop up truck camper?? "if" the unit has a HECO torsion bar lift system I'm very inspect the corners on a used one. It seems many of the MFG use the back top edge frame to mount the torsion bar Anchor assembly. And, they typically used plywood for the edge frame. Not "Should" but "When" the rear top corner edge boards let the Mounting bolts pull thorough the old/rotted/ stressed plywood frame it puts the torque on the roof top and corners. This results in the Box top becoming twisted, out of square and not parallel to the camper base side. Now , this causes the top to bottom seals to move and unseat allowing for water anywhere along the Top roof to Camper bottom seals. (Search for HECO Lift or Skamper roof repair in the forums for more info and lots of pics).
Because of this I'd look for water marks/damage along the whole top edges of the base camper's walls and esp. at rear corners. Other than That My new (to me) very old Skamper seems to have held up very well since it was made in approx. 1984.