Given that is on the bottom of the slide and it will only be out when you're camping I don't think you'll get any critters in there (unless you full time and the slide is out a lot). The larger issue with this, to me, is what that does for heating or cooling. You'll have to address that if you want to do any winter camping.
Before I would do anything to cover it, etc. I would watch what all the plumbing and wiring does as the slide moves in and out. Then design your solution to account for that.