We don't use our 2006 MP over the winter here in Norway. It lives in our unheated garage, where the temperature can drop quite a bit below freezing. I plug the van in about once every 3 weeks for 24 hours to keep the batteries charged - at least our model gets very upset when the battery has been flat, and it's not good for the battery either. On the advice of both the Amcar club in Norway and the Norwegian equivalent of the AA, I also start the engine on a couple of occasions during the winter and leave it running for about 15 minutes. The van has survived two and a half winters of this treatment!
If your van will be as cold as ours, don't forget to empty the waste and water tanks - and the pipes, you have to remove the fuse for the pump and open the tap to let the pipe drain into the tank and then out through the drain there. I also have a couple of reusable damp absorbing bags in the van, though I don't really know if it actually helps the upholstery.