After 3.5 yrs the seals on my Escalade crumbled. Had them replaced at great expense at Lazydays in Tampa. Drove 2 nights north and already could see rubber wearing off of the seals. (I assume the vibrating of the slide while on the highway causes rubbing but not sure) I called back to Lazydays and they told me to use Slide Seal Cleaner on them. I did and for the most part it seems to work. Comes in a spray can....you spray on and wipe off...available at any rv place. I use it about every 5 or 6 times the slides go in and out.

I buy Camco slide-out seal conditioner at Camping World. It's not terribly expensive and 1 can goes a long way. I treat all exterior "D" and wiper seals twice each year to keep them supple. If not, and particularly those exposed to sunlight for long periods of time (like wintering over with slides extended) will get mushy, gooey, and sticky, as though they're about to melt. I had to replace the "D" seals over one of my mega-slides and the price came out to something like $5/foot. Yes, very expensive. Note, do not wash the seals with soap, use water only and a soft brush or cloth. Let dry before applying the conditioner.

Talcum powder as well