The same thing happened on our first trip with our 320RS a few days ago. My wife extended the same slide and heard crackling sound and the black plastic piece on right side edge cracked into a couple of pieces and broke off. I looked underneath outside. On the cross bar extending across the length of the slide held on with bolts, the right end had dropped about 3/4". I tried to tighten up the bolt, but could not do much (probably too much weight with slide out). Dropped it off at the dealer on the way home.
Palomino Columbus 320RS
Chevrolet Silverado 3500HD