Our popup's owner's manual said not to cover it because it couldn't "breathe" so we decided to tarp it just down past where the roof met the camper body (when in the down position). That left the door available for some airflow, and it worked very well. We secured the tarp down with bungee cords. We never removed the canvas, and even at age 17 the camper's canvas never smelled mildewy. (We covered it for 12 winters; the first 5 we didn't, and ended up having to replace the roof due to water intrusion.)