Choosing a Vehicle
Different vehicles have different advantages for homeless living. RV's, vans, and trucks with camper shells are in some ways most comfortable. They are, however, obvious residences. Police will check them. People will notice them parked on their street. Finding a secure, suitable place to park in an RV, van, or truck can be a nightmare.
Cars, are easy to park but they are cramped. They're suitable for single living, but are not a very good solution for couples or families. Don't rule out compact cars just because they are small, though. Many compacts have been engineered to maximize interior space, and they can wind up being more comfortable than a full size vehicle.
What you need to look for in a car is a way to lay down flat and stretch out full length. I'm 6'4" so the roomiest back seat will not allow me to straighten my legs. If you can't stretch out full length, leg cramps are inevitable, and certain health problems like deep vein thrombosis (a potentially life threatening blood clot arising in the deep veins of the legs) can be caused by long periods of immobility. It is essential, then, to find a comfortable way to sleep.
When I am purchasing a vehicle, I look for front seats that can be laid down flat. I like to sleep in the driver's seat. Somehow it gives me a psychological feeling of control. The very best car for this that I've ever owned was a Honda Civic. The worst car, which I liked for other reasons, was my Datsun 280Z. Any sports car will be bad. Most commuter economy vehicles will be good; they'll have a sweet spot that will be comfortable. If the front seats are no good, how is the cargo space? Can the back seats be folded down to provide more trunk room? That's a bed. Station wagons tend to be pretty good, though too big for my taste.
An alternative to using a car cover is suggested by LiveJournal user, Crasch. He recommends buying a car, removing the rear seats, stringing a curtain just behind the front seats and blacking out the rear windows. It sounds like a viable method, and I recommend his article. Then the living area you've just created can be dressed up any way you like. There are two disadvantages to his method. The first is that it is obvious that he is doing this regularly if he is discovered. I'm not sure how bad that is, though. The police are going to cite you or they aren't, and deniability probably isn't a huge factor in that decision. The other disadvantage is a shorter warning time before cops or thieves can get in. I like to sleep with few clothes on, so I need time to get dressed before the invader is confronted. He makes useful suggestions, and the article is worth the read.