I went on a roadtrip this past summer from Pennsylvania to Utah and figured car camping was the cheapest way to go (a lot of walmart parking lots but saved me at least $1000 over two weeks).
I'm 6'1 so I needed more room, took out the front passenger seat and laid out a couple cut up memory foam pads from walmart out. Slept like a baby, unfortunately my tailgate refuses to open so it was a lot of side entry. Didn't have to take out back seats but it was a solo journey, so I had the other seat to leave stuff on. Def need to take out the front or make an elevated platform if you're 6' or taller, or wake up with a sore neck.

Making a whole video out of my instagram stories and footage, but here is just my car set up (sorry it was dark)

 

I slept in my Fit for two weeks last Spring in Washington State. At 6 feet exactly I noticed that I had to lay slightly diagonally across the back, with the front passenger seat scooted all the way forward and then the back tipped forward a bit too. I kept the care in full camper mode for the two weeks, so I had two sleeping pads (one narrow and stiff, the other thicker and softer), then two heavy blankets folded twice to to lie on top, and then a sleeping bag on top of those. It was plenty comfortable. Learned a few things though: bug screens for mosquitos that are somehow affixed to the open sunroof/windows/tail hatch are worth their weight in gold if you'll be in a buggy area. I slept a fair number of nights truly boondocking--truck stops, Walmart parking lots, etc. I found that I slept far more soundly and with less anxiety the nights that I pitched for a camp spot in a state park. For $15-$40 depending on the area, if you have access to one they feel much safer and you can use their showers usually (and a lot cheaper than a motel, and frequently closer to your destination). If the weather gets very cold, crack the windows a tiny bit. It sounds counterintuitive but the colder relatively dry air outside will make you a lot less chilly than the cold wet dewy air from being confined all night. Be sure to use something fairly solid to fill in the small gap between the back seat that's been laid forward and the front seat (I used a palett of water bottles and a cooler and it worked well).