WV Sharan - on demand camper under $130
Use the boot!
If it's not flat (as with my 5 seater) - use it for storage. Get some durable boxes that will hold your weight. If it's flat - skip to page 3 :)
Light, durable and flat surface to put your mattress on - plywood works great and is cheap. Make sure it's easy to put in and out
Car bed / mattress
Make it comfy but also durable. An air mattress might not last long, I suggest rebonded foam sheets.