With regards to your first inquiry, read these few recent discuss threads and make an informed decision on covering your coach.
http://www.keystoneforums.com/forums...ad.php?t=30266
http://www.keystoneforums.com/forums...ad.php?t=30597
http://www.keystoneforums.com/forums...ad.php?t=28097
As far as leveling jacks, are yours leveling jacks or just stabilizing jacks? Lippert makes a leveling system for travel trailers known as Ground Control which levels a coach vs the traditional 'stabilizing' jack that just provides stabilization, electrically or manually driven. Personally, if a coach is relatively level in a winter storage location, I would not engage any jacks. The tongue jack can be use to keep a slight pitch forward or backward in order to direct water runoff. But, I don't see a need to use jacks and neither do 95% of the other 1,350 RV's in my storage lot.