I did this for a year once. It was an '89 Chevy Blazer with tinted windows, the back folded completely flat. Granted, I was younger then, and it was in a temperate climate, but I didn't even use a mattress or sleeping pad and I had an airline pillow and a thin blanket -- that's it.
I found it awesome. No rent. No utilities. Saving that much money you can afford to eat out whenever you want -- but I rarely did. Mostly I ate raw things like fruit and nuts, and canned foods right out of the can. I also had a job where I could eat at least one meal a day. With enough looking around, you can find a public shower (mine was at a beach but you could use a YMCA or public pool). I had a network of bathrooms to use for shaving, washing up, and of course using the bathroom. Unfortunately peeing in a bottle was a necessity sometimes.
My favorite places to park at night were behind big hotels. No one thinks twice about cars parked in hotel parking lots. Mine was an "Extended Stay" so a vehicle parked long term was not a problem. You hear some really interesting things in those lots at 2 AM.
There are unexpected challenges but unexpected perks. Timing, looking presentable at work, what if people see the inside of your car (it will look like someone is living in it). But I was able to find most things I needed extremely cheap or free. I knew a guy who had a laundry room in his building that was accessible without a key, much cheaper than a laundromat. Keeping clothes from being wrinkled was a complex matter of folding, rolling, and hanging certain things on the suit hangers. Tinted windows really helped from being spotted. You can get fined for vagrancy. If you can't find a bath you have to use baby wipes -- which work surprisingly well.
An interesting twist on this was that the car had a moon roof that was stuck in the open position. Seriously. I had to rig up a tarp with ropes, which created a pool hanging down above the drivers and passenger seats I had to carefully empty after it rained. The rope broke the seal on the doors, so sometimes you'd get a nice cool shower when you took a turn too fast, down the back of your shirt.
Overall I've found older vehicles work better for this than newer ones, because people expect them to look like junk and no one thinks twice about a junker with -- for example -- a shoe sticking out back hatch to hold it just slightly open, for ventilation.
Having a tailgate is VERY nice for sitting on, eating, watching the sunset. I've sometimes fantasized about doing it again sometime, even short term. This time I'd use a full-size pickup and actually put a bed in the back and a propane stove with a custom-built counter, shelving or drawers beneath the mattress, etc. It's an excellent way not to care about anything in the world. You can go where you please, spend exorbitant amounts on things on a whim, put any money you need into your car because it's your sacred home space. You will need a curtain behind the front seats so no one can see you sleeping in the back through the windshield.
That said, I would never live in a camper. Too expensive and too damned convenient.