Your photo doesn't work. If it's on the inside top of the sliding door, about half way along, with a spring and a couple of white plastic wheels partially visible inside it, then it's a half-way door stop for the sliding door. For a camper van, it's nice to have because it means you don't let too much heat in/out and you retain some privacy when you open the door.
BUT it gets in the way if you're trying to create a nice clean interior line above the door. We sold ours and replaced it with one that
mounts on the slider track outside.