There are a large number of different types of molding inserts, but the two "typical ones" are called "vinyl screw cover trim" (typically used around doors and windows)
https://www.amazon.com/RV-Polar-Brig...+screw+molding
and "vinyl trim insert" (typically used on long "corner" molding and on horizontal trim runs)
https://www.amazon.com/Camco-25262-V...+screw+molding
The "vinyl trim insert" is more pliable, is easy to pull out of the aluminum molding strip, and will "shrink" over the years. Almost all of us with RV's older than a couple of years, will find several places where the ends of the vinyl trim insert is gapped and doesn't reach the end of the molding. Often times, RV manufacturers will use a staple or screw to "hold the end in place" so it won't shrink as much. Once you pull the old vinyl out of the molding, you'll see that the molding is secured to the side of the trailer with screws and the vinyl is acting as a screw cover (to make things pretty). It's available in 1/2", 3/4" and 1", so measure what you take out to be sure you order the correct size. Almost every RV dealership carries replacement vinyl trim insert in their parts department.
The much less pliable "vinyl screw trim cover" is much thicker, often times it will break if bent too much, and is used around windows and doors. They will often "fit in the other type molding" but aren't designed to be interchangeable.
It's an easy "owner fix", simply pull the old vinyl out of the aluminum molding, squeeze the sides of the new vinyl and "pop" it into the track.
Functionally, the vinyl only serves to keep water and dirt/bugs, etc out of the track and to make the "long run of screws" less noticeable......
The first picture is the vinyl trim insert and the picture with the ruler is the vinyl screw cover trim