As long as the roof is flat or only very slightly curved, I've had great success with Scotch VHB tape on aluminum roofs. It should work on fiberglass as well but it's just not an option on a rubber roof. I do use eternabond tape under the mounts on my rubber roof but just for additional sealing.
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I had considered SuperStrut as a way to make the panel brackets easily removable, but only considered whole struts. I like your idea of (apparently) cutting them into pieces. It's much less cost and weight and makes access to all margins easier. Positioning becomes less flexible but, in reality, I haven't moved them and probably won't.
In the final design I used basic Z-brackets screwed directly to the roof. The 3 mounted along the margin are screwed into the 1" square aluminum tubing that frames the roof, using stainless steel drilling screws for aluminum. All others use brass rivet nuts with 10-32 threads. (Supplies are from McMaster-Carr.) I couldn't attach the 4th on the margin because the Wi-Fi repeater is in the way. I may reposition it or, as it's sorta useless, remove it.
All attachments use butyl tape under and Dicor sealant over to prevent water proofing. After all panels were attached I threaded a stainless steel wire through all of them as security, should one get pulled off.
All that was a year ago. Since then we've put over 10K miles on the trailer and they're still solidly attached.
A disadvantage of attaching them directly is lack of access under the panel to clean the roof. I can take the brackets by detaching them from the panel frame but, once I do, access to the underside to reattach them would be extremely limited due to lack of clearance. (I originally attached the brackets to the panels before attaching them to the roof. Should I ever decide to detach them, perhaps for cleaning, I'll use small sections of SuperStrut as shown by Paul_R when reattaching, but that would require new holes. Hmm...