I have always had large estate type cars where you can put the back seats down and sleep in them. My present car, a Meriva, not quite as long as an estate but with more headroom, it is fine to sleep in. I take a sun bed mattress, sleeping bags, duvet, and pillows and it's quite comfortable. I take a small cooker so I can have a coffee in the morning. To be honest I feel safer locked in my car than I do in a tent, and I don't have a soggy tent to pack away if it rains.
Ilona