For most people, a caravan is a big investment, besides your house or car. So you don't really want to see it deteriorate any quicker than it should. The best way to protect it is with a shed, purpose made to suit. Height and size are the main things to consider, as well as a shed made from quality steel. While you're at it, why not make the shed a bit larger to also give some storage space. This means you can have some things safely in storage while you're travelling, but it can also give you the ability to unpack the caravan better when you return home.