I use a cover on my 33' fifth wheel. It's a pain in the &^@&* to put on, even with two people. Its breathable and water resistant. I've noticed, with previous coaches, that it helps keep the roof from oxidizing so badly over the winter, and accordingly, minimizes any streaks on the sides. Naturally, the coach stays cleaner as well.
It's a hassle, but I believe it's worth the effort.
Jeff, Laurie, + kids
Three time former Sunline owner
2011 Big Horn 3670RL Fifth Wheel
2012 Chevrolet Silverado 3500HD D/A DRW, Classy Chassis Conversion, aka "Garage Queen II"
Hensley Trailersaver BD3 Air Hitch