The wire has probably dropped down inside the rear cap. Tiffin puts a foam baffle across the back to keep heat from the engine from rising up inside the cap, so the wire won't drop all the way down.
There was a thread somewhere where someone dropped the wire for their backup camera, and they were able to fish it back up using a piece of stiff wire (coat hanger?). You might be able find the wire for the clearance light through the hole where the camera is mounted; but if it were me, I would probably try to run a new wire from the adjacent light. You can probably enlarge the hole under the clearance light somewhat as long as you keep it smaller that the light. Just be careful you don't knock the wire off for the second light.