I was right there with 2oldmen on the "just fill the crack" until I thought about it a little more.
So many on this forum don't like Dicor because it does crack after so many years. All you need to do is fill in the cracks with new and you're good.
What happens is the water goes into the cracks, as the cracks get deeper and wider the water works it's way UNDER the Dicor letting water enter the RV. So then members assume the stuff is no good.
You need to fill in cracks prior to any leakage, you waited too long. You'll now will have to very carefully remove the caulk and replace it with new. Attempt to find the maker of the skylight and see what they recommend. Some skylights will become brittle and crack if you use the wrong stuff.
If you have a rubber roof using any hardware suggested caulk can led to holes in your rubber roof. Rubber roofs and petroleum products do not mix. So the "handy hardware man" selling you a tube of cheap blackjack, may cause you thousands of dollars of damage when you need a new roof.