In reply to Georgert:
We've been to Greece a couple of times in our van, though not to any of the islands (yet).
We travelled through France, Switzerland and Italy to Ancona and took the ferry to Patras. The St Gotthard tunnel route through the alps is open in all but the worst winter conditions and is included in a Swiss motorway vignette (but its probably a good idea to have a set of snow socks/chains tucked away just in case).
The ferry from Ancona to Patras (Superfast / Anek Lines recommended) was very comfortable and efficient. (NB the "camp on board" option is not allowed during winter).
From Patras its only a couple of hours to Pirhaeus for ferries on to the islands. However as another poster has said, the Peloponnese is a great winter destination and much easier to get to. Check out Nafplio, Zobolo and of course Leonidio.
A final piece of food for thought - when we were in Leonidio we met a couple of climbers from Crete who had come to Leonidio to escape the rain in Crete! I don't know how typical that situation was!
Hope you have a great trip - and if you do take your van to Crete I do hope you'll post some beta on how it went!