Everyone has their own opinions regarding to cover or not to cover. Personally I prefer to cover and have never had a problem with mold or chafing. Different climates will present their own issues of course. My experience was that the Tyvek covers didn't last very long before becoming unusable. I'm now using a custom cover made from sunbrella fabric and it's held up very well so far (5 years). I also found that having zippered access doors was something that I wouldn't be without so I can make periodic inspections of the coach while it's in storage without a great deal of hassle. Hope this helps.
Al