Here's how i converted a Plymouth Voyager minivan into a travelling gipsie wagon, for almost no money at all. With a little time and an extended-bed minivan, you can do it too! NOTE: Although i haven't tried it, i'm sure this plan can be adapted to a smaller minivan as well. Use your creativity!
Step 1: Clear the seats.
First, you need to remove the middle bench seat from the van. In a Voyager or Caravan, this is done by pressing levers on the front legs, lifting the front of the seat up, then lifting up and back to disengage the back legs. Check the documentation for your van if you're not sure how. Store the bench seat somewhere out of the elements; as long as you keep the camper set-up, you won't be using it again!