The flip side is that a cover will also trap moisture in there on the milder winter days when the sun is shining and things warm up a bit, causing things to start rusting up. Our son in law works in residential heating and air conditioning and his advice to me has always been to not cover and in spring time give the AC unit a good rinsing with the garden hose. Been doing that for years, both on trailer unit and the house unit. Never been covered, and we get our fair share of snow here as well.
2011 GMC Sierra 2500HD
2014 Rockwood 8289WS, Diamond Pkg.