Hello and welcome!
Let me start by saying I think you have more experience that me in this area, I have never worked with fiberglass... but I will share my experience since I just removed my Delta about 2 weeks ago. I have a 2003 24RB.
I used a product called Thiox Thickened Epoxy Adhesive, something a boat builder friend of mine suggested. First I used a check of insulation foam to plug the hole, the kind of stuff you get in sheets at Lowes/Home Depot that is green or pick. I cut a round plug and jammed it in the larger hole good and left it inserted a bit so I could see the fiberglass level. On the inside I has some softer foam (like you might find in a puck and form hard device case) and I placed that over the hole and held it with some tape. I then bought a $18 LED light (battery operated) from Amazon and mounted it just to cover up the place the old antenna crank was located. And now I have a cool multi color LED that lights up the interior on 3 C batteries.. so I can save a little on the coach batteries.
The Thiox once hard can be sanded, drilled, painted, etc. Its easy to work with and appears to be pretty solid. I have used it a few other places on the fiberglass to fix minor issues. My plan is to sand a little more (I did a 2nd coat since the images to make sure I was overlapping the seam) and paint... I might even coat the area with some Dicor EPDM Self-Leveling Lap Sealant in the end (which would just be overkill but give a little safety), but so far I have not seen any evidence of leaking from the Thiox. I plan to maybe install a solar panel here in the future... and not too many folks climb on the roof so I didn't think it needed to look perfect. Here are a few images:
I hope this helps.