Being in South Bend, we get so much rain it makes Seattle look arid. I use discrete waterproof wheel covers, then a full body semi-waterproof cover, then the lightest-weight waterproof tarp over that, but just enough to cover the roof area. For my Minnie Winnie 22M, I use a 12'x30' 7 mil tarp, anchored down with Rhino bungee cords and adjusted to length with lightweight rope. The goal is to cover the roof area, and the sloping nose of the vehicle, thereby shedding the bulk of the water away. Your main cover lasts longer, and the lightweight tarp can more easily be lofted over your RV and is far less expensive to replace.
2019 Minnie Winnie 22M on an E-450 frame