I recently replaced the skylight over my shower and also went ahead and scraped/resealed the entire roof of my coach...
I replicated what was on my roof and used BOTH SB140 and Dicor lap sealant. The SB140 obviously had a large bead sealing the mating surfaces together. There was SB140 where the plexiglass material touched the fiberglass of my roof. THEN the edges were sealed with Dicor. That is how it was, so that is what I did. No issues thus far.
There is a great YouTube video on this as well, and their situation matched mine.
BTW, resealing my roof took 11 tubes of Dicor, glad I bought the case at a discount on Amazon.