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The simple fact is that cars make pretty darn poor beds. Sure, they will work in a pinch, and I suppose the larger the car, the more likely you are to have enough room, but really, unless you are a very small person, you're not going to be able to get very comfortable laid out on a bench seat, or reclining in a bucket seat. I've got a suv where I can fold down the back seat and stretch into the cargo area, and its still not something that is very comfortable.
If you go with a minivan or an SUV, you'd have a little more room, and that might work, although, if it were me, I'd take the extra money you'll spend for an larger car/suv/van and use it to just buy a motel room on the rare occation you can't find or don't feel like staying in a campground.