I hear you on that....I am thinking to bolt onto the fiberglass as you would with a steel body, BUT also add a plate of steel (3/16"?) along the inside as backing for the nut. Basically sandwich the fiberglass rock between the steel plate and the rocker rail...
But would hate to spend $$ to find out my idea was wrong...so hoping someone can share their actual experiences and pictures as to what they did successfully.
I really don't know much abouut your setup but I can't imagine being able to bolt anything to fiberglass and not have it crack. Yes you can put a plate reinforcement behind to help but that means it will just take a little longer to crack at the edge of the plate. Lots of leverage and weight when you stand on it.
I would bolt to the frame.
If you always do what you've always done. You will always get what you've always got.