Also, you might want to go up in screw diameter and use a more agressive thread pitch. I recommend SS screw too, the ones that come with the awning eventually rust. 

   This is what I mean. Each bracket has five screw holes and only one screw on the bracket to the rear of the slide-out actually grabs something solid in the wall. Once the spring tension was pulling on the brackets the bottom screws pulled from the wall just a bit. So, I'm wondering if it's possible to use some kind of anchors in these type of exterior walls. I'm quite familiar with screw anchors, but I'm not sure which kind would work best in these walls.