The first consideration, of course, is whether your particular model of vehicle has enough room to actually stretch out and sleep. I'm six feet tall and have no problem sleeping in the back of my Nissan XTerra. The best way to find out if you fit is to crawl in there and lay down! I've found that sleeping in the vehicle itself has advantages. The foremost of these is the security of being in a vehicle during a rainstorm or foul weather. Also, while I'm not averse to old fashioned tent camping, I find that the vehicle is much warmer than the cold ground. Another plus is that I don't roll over in the night to find a tree root stuck in my back
