Best Product To Glue and Seal a Large Hole on the Roof

I wouldn't use Dicor under the Aluminum sheet since your roof is fiberglass. I would suggest using Proflex Sealant. It is a urethane based silicone that creates a strong bond along with sealing properties. Dicor is meant to be a sealant and not a bonding agent were as ProFlex covers both bases. The Eternabond tape is a good suggestion. Then use the Dicor to seal the edges to keep it from peeling over the years.....Better overkill when it comes to leaks. You can buy all these products at your local RV Dealership