We just got a plain white cover from CW, about $15. We adorned it with a SC state Palm Tree and Crescent vinyl sticker from this place:
States Hobbies Gift Ideas | States Hobby Gifts for Men & Women
It's in its 5th year, second trailer, no problem. When it gets a little "mildew-ey," I spray it with the same Spic/Span and bleach water I use on the roof, and it cleans right off. The sticker has held fast, no problem, no discoloration.