My first stent in the vehicle life was a summer inside a Suzuki SX4 hatchback. Not even enough room to lay down; I ended up building a platform from the dashboard to the back hatch, over the seats, in order to get horizontal. My head rubbed the ceiling when I rolled over.
It's very possible to live out of a car. I know several people who live out of a Prius. I upgraded to muh big-ass van because I wanted certain amenities like standing height, my own shower, refrigerator, etc. But, if you're willing to go off of baby wipes and spray bottles, or truck stop showers, or the nearest river, you should be able to maintain some resemblance of hygiene whilst car traveling.
I have three first suggestions:
1) Figure out a bed. Remove seats if you have to.
2) Either add a supplemental 'house' battery, charged off your alternator, or replace your engine battery with a marine battery and automatic cut-off to prevent depleting it and leaving you stranded. This will allow you to charge electronics and make use of other small loads, such as lights at night.
3) Get a simple two burner propane stove. Use it for cooking, use it for heat. When heating, cover the flame with a clay pot or cast iron pan for heat disbursement. Be conscious of ventilation. Burning propane produces both carbon monoxide and moisture.
