This YouTube would be a good place to start.
Sometimes when our slide is out a long time the secondary seal will start sliding out, the one on top has almost come all the way out before I see it
Sounds like yours is gone or in very bad shape.
I recently replaced most of ours with this.
https://www.amazon.com/gp/product/B08F69QN73/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1