If you are talking about sealing around the light fixture and out onto the fiberglass, this what I use. http://www.amazon.com/3M-Marine-Silicone-Sealant-3-Ounce/dp/B000R4UB08
Just make sure you clean the area good with alcohol.
Regular silicone isn't recommended. I'm sure others will have opinions also .
ProFlexRV (made by Geocell) works great for all those outside RV caulking issues, except on the roof, where Dicorself-leveling sealent would do a better job. Camping World sells it.