I am doing this at the moment, but probably on a more luxurious scale. Last year I went on holidays for a month in my little Mazda 121. It's seats fold back flat, and the front seat (I used the passengers seat) can be moved forward, so when it folds back it tucks in level with the front of the back seat.
The car I have now is bigger, but the back seat folds 60/40 forward. I measured it when I was buying to make sure it was long enough. I have made three box cushions from dense foam to fit the area I sleep exactly, and these give me a wonderful mattress. This gives me a better sleep than I would probably get in a tent. I have a tent with me, but I think I won't bring it in the future unless I am sleeping somewhere for more than a few nights.
Sleeping in a car is much more secure. It can be centrally locked, so no boogey men can get in. In Australia, wombats are known to rip huge holes in tents with food in them, and snakes have been known to nudge zippers open so they can get in (always leave your zipper tab on the outside) - unfortunately most of our snakes are venomous, and we have something like 8 of the 10 most venomous snakes in the world.
The two main problems with sleeping in the car are moisture and security of tenure. When we sleep we expire, so moisture builds up on the inside of windows - even if you sleep with them cracked open. When I have located a nice camp site in a national park, I set up there, but if I drive during the day, someone else usually takes it - that is why a tent would be good.