RV roof vent
You’ve probably seen these things on top of buildings without realizing it. This is actually a roof vent or a mushroom roof vent. You know, those spinning domed objects on top of homes and offices? They remove heat and undesirable particles from indoors by creating a suction that pushes them outside.
This one just happened to be on top of an RV, and it seems to be in pretty bad shape. Modern RVs have enclosed roof vents to prevent moisture and outdoor debris from getting in. As for this one, the RV owner might need to get it replaced or at least fixed to keep water from leaking in.