It is quite common for that plastic trim to become brittle and crack and eventually crumble after a few years. It absolutely has no functional value, like, sealing for water intrusion. It's only aesthetic to cover the screws inside that track.
Here is what you need to replace it:
Click here.You can purchase this stuff from any RV dealership part store or on-line anywhere.
It comes in 100 foot rolls, is very easy to install. Just pinch one end and slide it in the track, keep it pinched and just slide your fingers down the track. It will slip right in.
The only spot you may have to work with is the ends. It really needs to be tucked under your metal trim pieces.
Trying to fix the broken stuff is futile. It will continue to crumble. While you are replacing it, do both side of the trailer, and if yours had the same stuff vertical, on the ends of your camper, replace it also. 100 feet will be more than enough to do your entire trailer.
No, you cannot save what's beginning to split and crumble. It will only get worse. It's probably better at this point to go ahead and pull the existing completely off, use a piece of it to take to your RV parts store so you'll get the correct width (they do come in a couple width sizes) and replace it all at the same time.
If you don't, it's just a matter of time till wind while driving will catch it and pull it out of its track. Then it will be flopping against the side of your camper, maybe even break and beat the side of your camper. You don't want that to happen. Better to get rid of it before that happens and have nothing for a short while than try to save something that is now dead and save it.
It's easy to replace, and depending where you purchase it, not expensive at all. Your camper will thank-you for the face lift even for those pieces that have not started to split and crack up.
