You will probably get a bunch of opinions as to what to use to treat your rubber slide seals and flaps.
But, hands down, the best is 303 Aerospace Protectant. It was originally formulated for rubber and vinyl, only, and it works.
Spray it on your inner and outer seals and flaps, don't forget the top and seals under the slide.
Also great to use on vinyl decals and graphics.
Steve & Cheryl + Charlie & Casey, our furry kids.
2012 Forrest River Lexington 283ts
Toad, 2013 Subaru XV Crosstrek