I purchased a temporary shed, sometimes also called a portable garage, from Costco for something like $250. The whole trailer fits in there perfectly, and it is much better than a customized cover or tarp of any kind. It provides nice ventillation, easy in and out access, and sheds the leaves and snow much better. It is more expensive than a tarp, but I imagine cheaper than a customized cover. But some people have a problem with the looks of it in the yard. I don't have a problem with it, and kind of like it knowing that the RV is cozy inside.
Honda Odyssey Touring 2005