These brackets are the way to attach a roof rack to a camper shell. You are asking for trouble if you get one of those racks that are attached to the top of the shell with screws.
Your roof racks won't work their way off these brackets because the lip on each end will prevent that. These brackets come with 1 inch stainless steel bolts. If your shell has that so-called insulated roof, you will need to buy 2 inch stainless bolts to deal with the thicker shell and be careful to not over-tighten. A brad point bit is best for drilling the fiberglass, and you will find that the fiberglass wears out the drill bit quickly, it will be pretty much expired by the time you put four brackets on. Spend lots of time with careful layout work. Once you drill holes in the shell you can't take them back out. Check that the surface where you want to mount the bracket is flat. I had one slightly curved surface on the shell where I wanted to mount a bracket, learning this after I had drilled the holes. I had to cut and weld the bracket to get a slight bend in it so that the gasket would seal against the shell properly on both bolts. This problem occurred because I wanted the rear rack to be very close to the back of the shell to facilitate loading a boat on the roof rack.