Mine has always spent winters uncovered in a grass field storage lot. It's in a row of other RVs so the sides are protected a bit from the elements. I place the wheels on 2x8 boards and cover them with tarps. I leave it tongue low with all batteries and propane tanks removed and the roof vents (with vent covers)slightly open for ventilation. Never had a problem.
I have seen problems with covered RVs. First, the covers don't last more than a couple of years. They flap in the wind and rub against the RV leaving marks and a dull looking finish. I have also seen mold and mildew from covers that don't vent properly.