I'm a member of of a boating forum and thought I'd copy your question, post it there, and relay responses to you.
What they generally say about 5200 is be careful NOT to use it on something you may want to take apart again. Other than limitation, a strength (no pun) instead of a weakness, I've never heard of 5200 not working.
My only personal caution would be to be sure the bonding surfaces are not contaminated with something else that's already been used there. As an RVer, you already know how notorious silicone is for rendering a surface un-bondable. I doubt you used silicone there, I'm just making an example.
FYI, 5200 is a one-part product. Usually once opened, the rest of the tube goes solid in a very short period. Pretty much no shelf life. Some of the boating guys have had a little luck packaging their open 5200 (and the less aggressive 4200) using a Seal-A-Meal vacuum packer.
I think I'd try to expand the bonding surface of the fiberglass some way. Hard to say w/out a picture, but something like bond an aluminum strip or plate to the 'glass with the 5200 then attach that to your existing brackets. Old signs are a great source for DIY aluminum!
I'd just looked at the pic of your rig in the FSD thread, and it made me about sick that you were having that cabover issue. Want to help any way I can.
Let me know if you want me to post your question.
