Check your RV's manual to see whether its roof is intended to support a person's weight. If not, place a ladder as close to the RV's side as possible and carefully climb until you can easily work on the leaking skylight without leaning too far.
Use a putty knife to scrape away the existing caulk surrounding the skylight. Removing the caulk will take some pressure, but be sure that you do not press hard enough to damage the screws or rivets that hold the skylight fixture in place.
Pour a small amount of paint thinner or mineral spirits onto a rag or soft brush and rub away any residue from the original caulk. The cleaner the skylight is, the better the replacement caulk will be able to adhere.
Insert the tube of roofing cement into the caulk gun, its pointed nozzle facing away from the gun's "trigger." Snip off the tip of the nozzle with a sturdy pair of scissors or utility knife.
Apply an even ring of caulk all the way around the seam where the metal or plastic flanges of the skylight meet the RV's roof. You do not need to constantly pump the caulk gun. Rather, a more uniform flow will be produced if you squeeze the trigger at a constant pressure until it cannot be pressed further. Make sure that the caulk completely covers any gaps between the skylight flange and roof.
