As you can see from my signature block, I also have a Phaeton. Because of the low quality clear coat paint they use, I would strongly recommend not using a cover at all. Otherwise you'll probably wind up with a bunch of cover scratches. I have had mine for almost four years alway outside, two years in northern Idaho and two years in northern Nevada. Keeping it waxed will keep the exterior in much better condition than using a cover.
Bob, Sherron & Kinsey (Goldendoodle)
2009 Tiffin Phaeton 40QSH
2014 Jeep Cherokee Limited