And I'll bet that even less than 1/3 of RV owners regularly wax their RV. While it is true that most of the RV's I see in storage are not covered, they are much more dirtier and hotter inside than mine.
To the OP - I use an ADCO cover, and it is starting to get thin in places. I don't know what Sunbrella is, but if you want to cover your RV, and plan on always doing so, then it may not be a bad idea to invest in a quality one. I don't think I'd buy another ADCO - but it does keep my TT clean and cooler inside while stored. Make sure you can get into it via zippered access while covered.