Rub Rail,
Paul has the answer there. Last year I couldnt figure out why the rub rail was coming out of the slide, in the spring. Thought my neighbour must of given me a rub while he was docking. Then while trying to place it back in the slides noticed a lot of dirt and moisture in the slide.
Of course,,,,,, water/snow gets in there,, freezes and pushes the rail out of its guides. I now take the end caps off in the fall to let it drain.