You can convert any large van into a small camper. Some auto dealers even sell such converted vans to people looking for an RV in disguise or just a compact mobile home. You may not have all the amenities of a full-size camper, but you can install a few pieces of furniture to transform an ordinary van into a simple camping vehicle for one or two people.
Remove all the middle and rear seats from the van. Some vans have rear seats that can be folded back into a bed, but removing those and creating your own bed will take up less space. Now you must deal with the holes left in the floor from removing the seats. You can staple strips of carpeting over each hole, or you can cover the entire floor with new carpeting.
Unless you have the carpentry skills to build a small bed frame, your bedroom can consist of just a sleeping bag. Put foam mattresses or an inflatable mattress underneath the sleeping bag to make it more comfortable. If you set up the sleeping area right behind the front seats, you'll have plenty of room for additional furniture and features.
The kitchen can be as simple as a small camping stove on a table. Fasten the table to the side wall with bolts and brackets. Attach side panels to the legs so you can safely store the propane canisters. You can also add a microwave or toaster oven on or under the table. A propane stove must only be used in open air, so install the kitchen near the back door, which you'll leave open when cooking.
Installing plumbing inside the van is very complicated, even for a small sink. The simplest solution may be to construct a kitchen sink by cutting a hole in a small table and resting a stainless steel bowl in the hole. You can pour water from jugs into the bowl and remove the bowl to dispose of dirty water outside. Constructing an actual rest room may be too ambitious because of space restrictions and the need to install a septic tank. Public facilities on the road can suffice.
Add any other amenities like a TV or stereo system where space allows. Bolt RV cabinets to a side wall to store electronics. (You can also use such cabinets to store foldaway tables and chairs to use outside when parked.) Any electronic devices in the camper will need power, and deep cycle batteries are your best source. Stock two or three such batteries and connect each one to its own power inverter. Build and install boxes in the van to store the batteries; make sure you have easy access to the inverter sockets.