First, the bottom line as posted above is that you do not want a busted water line ever. Draining, blowing, RV anti-freeze are the solution to that.
Covering, Rule 1 is that you must have air circulation in the camper. So don't tighten it up so no air get in, otherwise mildew can happen. Living in MN usually means snow and now usually a rainfall or two. I used to cover, but we leave in the winter sometimes so that was a disaster. The top froze to the roof of the RV.
I guess that the person with pine needles and branches is kind of right, but if you don't have trees, as we don't have, it is okay not to cover, at least in my opinion. Of course if you cover, there is less chance of water getting in or creating a problem. Remember, RV sales lots don't usually have trees and they of course do not cover.