I had the very same exact problem with my other cover... dealer had nothing... took it back to the manufacturer and nothing..
They finally gave me a set of "slide out slickers" . . They are plastic runners that are about 40" ;long and about 5 " wide. They have little dimples that face down into the carpet near the slice entry.
When you bring the slides in, you first place 2 of these at each corner of the slide, then bring in the slide.
The slide rides up onto them and any black marks are now on the slickers, and not carpet.
The black marks are usually caused by the extra adhesive that they use to put the corner guards on the bottom so the slides.. that oozes out from the weight of the slide rolling in and out, and that ooze traps grease and dirt.
K-Z paid to have my carpets cleaned, then i used these and never had another problem for the ba;lance of 6 years till i traded it.
Try looking here -- http://www.lci1.com/slideout-slicker