Usually a caulking is applied between the vent flange and the roof's surface when it is installed. The vent takes quite a beating on top, whenever you're at speed with your rig. A tractor trailer passing in the opposite direction, forces a wind equaling almost double your speed.

The vents will flex and move about, pulling on the roofing and eventually often breaking the seal. Clean off any old loose and dried caulking, that was applied externally since the trailer was manufactured. Apply a new polyurethane or elastomeric sealant and you should be good to go.

On the inside, spray some light oil on the cross rod that rubs on the cam tracks. Make sure the vents are moving easily, and they lock in the centered position detent. If they don't, water can leak inside while driving or whenever a wind drives the rain.