Water. That's it...just keep the water out.
Don't assume that the factory self leveling caulking on the roof is done right. Smart money is to Eternabond the seams and joints...or reseal if there is any question. Silicone used on side panels often pulls away from one surface without being noticeable. The factories usually apply a pretty little bead that is really only a surface application and does little to actually seal the joint.
Keeping water out just isn't as easy as it sounds with all of the flexing and movement of the frame and structure while towing. Takes constant checking.