Don't use one. Trailer is a 2012 model purchased in 2011, and for a few years it was in Moab UT so snow was not a big problem, just sunlight, UV and high temps. In the fall of '14 moved to Idaho, still have sunlight, UV and decent high temps, but also massive amounts of snow and trailer looks good.
There is some delamination to the roof but not cause of the weather, but other than that and a patch where a branch fell on the roof, it looks great. Yes the decals are starting to fade. And it sits in the driveway when not on the road.