Hi Morganic,
There was a recent thread which addressed the wet aspects of this issue, it is worth looking at -
http://www.practicalcaravan.com/newforums/fm_messages.asp?FO=1&FM=473231
One common suggestion is to open all the locker doors to ensure that air can circulate.
Make sure the van is secure from rolling, with the steadies and wheel chocks and then release the handbrake so that it does not seize on. Give your electrical connection plug(s) a good spray of silicone based lubricant and make sure they are protected from the elements.
If you leave the wheels on, it is a good idea to rotate them every few weeks so that a different part of the tyre takes the load. There are some people who take the wheels off, putting on winter stands. This is a good idea, but do check with your insurance company first as some policies explicitly require you to have a wheel clamp in place.
Consider taking the battery out and storing it in your garage (not on the concrete floor) where you can give it some care and attention with a charger.
Hope these ideas are of use, I am sure others will add to them and, knowing this forum, challenge at least one of them.
Cheers
Mike