Bike rack for Amara 450/2

Hi all. I have a Fiamma Carry-bike Universal bike carrier. It fits to the back of my Amara 450/2 and the top arms clips into the rails that goes across the back (2/3rds of the way up) of the caravan. Im not too happy for it to just sit in the rail (one side came out once !) and it was supplied with 2 brackets to fit to the van that these arms fit into. My question is if I wanted to screw these brackets in directly above the rail what would i find behind the fibreglass outside wall ? would it be wood ? I want to ensure I get a proper fixing.

Alan