cable slide out grommets
I think they serve two purposes...they act as a bumper, and they keep the cable from popping out of the hole.
I had a Keystone Laredo 330RL travel trailer before the Montana. It had cable slides and I recall a thread on the Keystone Owners Forum discussing these rubber grommets that I found very helpful. I don't think I can provide a link to the thread, but the screen shot below shows the thread title and author so you can search for it.
Mikendebbie Austin TX - Aggie '77
2019 Chevy 3500 High Country DRW
2018 Montana 3921FB
Three cute Aussies Gus+Fern+Jimmy