We have two motorhomes and leave one of them covered all winter. I bought a good quality breathable rv cover which covers the entire motorhome. I then bought a blue plastic tarp of a size that covers the roof and only comes down over the sides and front and rear cap about a foot. The tarp does NOT cover the windows.
I leave a roof vent open(it has a vent cover over it). We get quite a bit of snow, wind, and rain and I have never had any mildew or mold because of the breathable cover and the tarp not covering the windows. We also have never had any chafing on the motorhome since the tarp only comes down about a foot over the edge of the roof. You could leave a window cracked if more ventilation is wanted. This method has worked for several years for us without any problems.