If you have a fiberglass roof, I would remove the old skylight and discard it. Clean the roof area by removing the old sealant and dirt with a heat gun and scraper. I used one of those vibrating blade tools on mine. Screw the new skylight down to the roof every 3 inches with flat head screws countersunk into the skylight. Then clean the skylight and roof with acetone. Then apply Eternabond tape, starting from the rear and working forward. The tape should cover the screws by at least 1 inch and more if possible. The roof should have 2 inches of tape to prevent any moisture from getting under it. The key here is to make sure the Eternabond tape is put down on a solid,clean surface and roll it down to ensure it bonds. I did this on my MH and it has been through some very intense storms with no leaking. When I replace the skylight, I will cut the tape around the skylight and remove the skylight. If I can get the old Eternabond tape off, I will. If not, I will replace the skylight and apply Eternabond tape over the new screws and the old tape. I used 4 " tape the first time and will probably need 6 " next time. I used this tape on all my roof seams and am very impressed with it. I hope this helps.
95 Newmar Kountry Star, Spartan Chassis, Venice, Fla