I reworked the sliding windows in my Trailswest Campster
. It was certainly a learning curve for me but not all that bad of a job once I figured out the tools and steps needed for the job.
To do this job I had to take the window out, set it on a padded table, drill out the rivets that held the frame at the joint. I used a bar clamp that has the ability to spread items apart and then rotatae the clamp head back in the other direction to pull it the window frame back together. I had to slightly spread the frame apart to be able to remove the glass panes. Then cleaned out the old material and replaced it with new. You will have to put a new rivet in. That was the trickiest part of the job due to the interior shape of the frame extrusion.
The keywords you need to find the felt track material are "flexible glass run channel". You might have a beaded top edge or the unbeaded style which adds another keyword to your search. You will need to know the width and depth of the track channel in your trailer in order to purchase the right size.
The original material is a wound around wire with fabric. If you can find that type get it. It conforms the best to the rounded corners of the window. Plus it also lets any moisture that gets in drain through the cloth and into the channel where it can exit through the weep holes. The other kinds are rubber which is what I ended up with. It worked but I had to do some trimming of the profile at the bottom and also cut some slots in the rubber so the water could drain out of the felt channel piece and into the track so it could exit through the weep holes.