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!
Step 2: Prepare the floor.
Next, put a tarp down. This is the voice of experience speaking, trust me! Even if the carpeting on your van's floor isn't primo, a tarp will make clean-ups *so* much easier. Just lift the corners, fold, then tip into a garbage can. Cheaper than vacuuming! Now it's time to start building the bed.
Step 3: Building a bed: the platform
For this, you will need a minimum of four milk crates. You *do* have old milk crates lying around, don't you? Sure you do. Or someone you know does. *Don't* use store-bought 'storage crates'; they aren't strong enough to support the weight you'll be putting on them.
Place one crate in each corner of your tarp 'floor'. i have an extra crate in the middle back pictured; in fact, by the time this build was done, nearly the whole floor was filled with crates! But my previous van had only four, and that worked just fine.