Mounted the rail for a slide topper by inserting 10+ screws into fiberglass above the slide. Scary! But I used Butyl tape behind the rail, and then added dicor as caulk at the top of the rail so no moisture would get behind to rust out the rail from the back even though the Butyl should seal the wall.
Yeah... I'm paranoid when it comes to water penetration and damage.
Rig: '16 XLR Hyper-Lite 29 HFS > 2014 Nissan Titan Pro4X 5.6L
Toys: '08 HD Screaming Nemo Ultra > '07 HD Dyna Street Dori
Family: DH - JC, DW - Malinda, DD - Sparky the Wonder Chug (Chihuahua & Pug)
Days Camped: 2016 - 22 (+13 reserved); 2015 - 27; 2014 - 16