For a slightly sideways look at this:
anyone with a slideout have any experience with them once they get covered in that wonder fine desert dust? I learned the hard way that the bed of our trucks stop dust from getting inside just about as well as an open window does. I'd worry that the tracks of those slideouts would junk/gum up and get awful sticky/gritty in no time.
OP you mentioned a soft cover... maybe consider going with a paneled cover instead? I have the FoldACover, and I hear BakFlips are awesome too. Basically you can fold back one or all of 3-4 panels giving you access to the entire bed if needed. Seems like that would solve the problem just as well without giving up cargo space to the rack, without added weight and without any longevity worries?