Hi Angela,
I think it depends on where you park your van. If you park under trees you can get sap dripping of the trees, bird droppings & this time of the year leaf stains. In which case it might pay you to fit a cover, or as some one sugested not so long ago a cover that just fits over the roof.
If like me you do'nt park near trees leave it, the choice is yours.
There is a company that sells polish at markets & caravan shows that say wash your van at the end of the season, dry it then apply their polish all over the van & leave it till spring, then buff it up.
G.