Two travel trailers ago, I had the same problem with my slide outs. I learned the HARD way to NOT use those RV rubber seal treatments, foamy stuff out of a spray can. Just like yours is pictured, the rubber began to deteriorate after about 3 years and every year it got worse. No amount of cleaning the rubber stopped the deterioration.
I had that camper for 7 years and when I traded for a new one, the seals were pretty much completely trash. They eventually stuck to the side wall of the camper and glued themselves. When the slide was opened, the rubber actually ripped into shreds, long shreds still stuck to the wall of the camper leaving holes and rips.
Because I sold the camper to my dealership in trade-in, I never attempted to fix the seals. But when that time came, they were pretty much all in great need of replacing. They never did leak water through. I suppose there was enough rubber, it still maintained a good seal when the slides were in or out.
The next camper I got, I did not use anything on the rubber seals except baby powder. On another forum years ago, it was suggested to use baby powder on a soft cloth and just wipe the rubber.
And on my current camper (now another 7 years), I've never put anything on the rubber seals on the slides, except when washing the camper. And that was whatever soap I was washing the camper with.
My seals are still soft, pliable, and not breaking down, leaving marks on the wall, ripping, or tearing. The camper is now 7 years old.
If you have already used that RV rubber seal cleaner spray foam stuff, your seals are already damaged and will continue to deteriorate, leave marks, and eventually start pulling apart in shreds. The damage is done.
The best you can do to prevent a replacement as long as possible, is to wash the rubbers with a dishwashing liquid solution. Dish soap is designed to cut grease. It won't penetrate the rubber and remove the damaged rubber (which is forever damaged), but it will help from sticking so bad to the wall, provided the slide outs are not closed in intense heat for a longer length of time.
You asked? That's my hard-knocks, hard-lesson in life story, all true.