I used the ProFlex sealant from Camping World listed above to reseal the front and rear cap areas on my coach back in 2005. They haven't leaked since. I check periodically to see if the sealant is still pliable, and it is. It seems to adhere very well to the fiberglass.
I agree with Dale, make sure the area is clean and dry. I used alcohol as well. In my case the old caulking on the front cap pealed off very easily. (One reason it failed, no doubt!!)
'04 Alpine Coach 36' MDDS