If you have a hatchback, and back seats that fold down flat, you can put an REI 3.5" thick camp bed diagonally across the back of the car. This can be surprisingly comfortable. Maybe I find it so because I was previously camping in forests, in a tent, and I'm now used to this kind of bedding, but I think the REI 3.5" thick beds are just pretty comfortable. I have a size XL, which I got for tent camping. I definitely prefer it for that, but in my car the 6 foot 6 inch XL is a bit too long for the space I have. I've "made do" by deflating the bed a little and bending it where it hits the back seat. If I had bought the bed solely for using in my car, I might have gotten the Regular 6 foot size. It's a lot narrower though, and when camping I don't like that.

You do not need a pillow. A polyester fleece works fine, folded up under your head, and you can also wear it if it gets cold. You can fold the fleece to any thickness or thinness you like. Ergonomically, this level of control is actually much better than a pillow, making one wonder why we insist on pillows.

The key to making the back of your car sleepable is not to carry too much stuff with you. You need to get things out of the way to sleep. You can shuffle a few things around, like a duffel bag or folding chair or some such, but try to keep it to a minimum or it'll turn into a PITA.

If you have a dog, he gets the front seat. Or the front driver's foot well, which my dog prefers because it's cooler down there. If you have annoying hard plastic bits between your 2 front seats, cover them with a towel so your dog can sack out comfortably.

My car has a metal body and thus gets fairly warm. The body heat of man and dog is definitely enough down to 40 degrees at least; haven't been in colder yet. Also those REI camp beds are well insulated. You will want a cheap sleeping bag. I got a Coleman so-called "30 degree bag" (yeah right) for $30 at Walmart. It's nothing special but it's fine for the car.

Your dog will argue with you about the heat level in the car. His method of arguing is pant pant pant pant and it's very effective. Roll the windows down if it's too warm for him. That's why you've got your sleeping bag. Eventually the car will cool down at night and he'll stop arguing. Then you can roll the windows up, leaving a crack for air if you like.

Cracked windows do allow noises in though, such as the urban street noise where you may be parking. It's a tradeoff that you have to decide. Cold and gloomy weather is better for car camping because (1) the dog doesn't argue with you as much, (2) people are glum and cheerless when they go out, so they don't make as much noise, (3) the damp seems to muffle sound. Yes, muffle all those people and sounds. Rah rah Seattle weather!

Parking in shady places is a good idea if you're carrying a dog, or a cooler, as the merciless sun will fry both. I do have "solar shades" for the front and rear windows, which also offer more privacy, but they are not enough to counter direct sun.

Park in front of structures that people are not "possessive" about. Like a big impersonal apartment building. Don't park in front of single family detached homes. They get irritated that you've parked in front of "their" part of the street, and they're more likely to call the police. Park in cities where people don't care that you're sleeping in your car. Don't park in suburban towns where the cops have nothing better to do than "look for crime," such as people who sleep in their cars.

The price of hotels is ridiculous. You may think a car is cramped, but a certain kind of "rugged" or "expeditionary" type person simply doesn't care. Your only "rent" is gas, which depends on how much or how little you want to move around. It's a convenient way to see a big country like the USA!

I hear that people in Tokyo rent "coffins" (?) just to take a nap for a few hours. The back of your hatchback is a lot roomier than one of these coffins, so do you feel weirder than some Japanese? When I'm lounging in the back of my car with my laptop, I think I'm pretty stylin'. It's about as comfortable as any setup I've devised indoors.

ianguitar, I drove all the way across the USA from North Carolina to Seattle because I was bored with suburbia and I had stuff in storage that I hadn't dealt with for 4 years. I meant to do a lot more tent camping in the woods, but hunters in the western USA started shooting bears in mid-August. That surprised me as they don't do that until mid-October in the mountains of NC. I guess the western forests are that much colder. My dog is named "Kodiak" and looks like a little black bear, especially if he's only poking his head up above some bushes. So later for that! That's how we started doing the urban camping thing, because I wasn't done with the storage unit yet.

nicewinoa, the easy way to take a shower in summer weather is to put a gallon plastic bottle of water out in the sun. Wait a bit and it will become warm. Then pour it over your head and shoulders with one hand, washing with the other hand. Switch hands as needed. Thankfully I'm not a sweaty person so I don't need many showers. When I do, it's not a big deal.

More points on car camping:

If you try to sleep in your car and it's cold outside, your car may turn into an icebox and wake you up. Last night I parked somewhere in Montana. I cleared out the back, crawled into my sleeping bag, had my warm clothes and hat on, etc. Woke up 4 hours later in an icebox.

If it's 25 degrees Fahrenheit outside, that's what happens in my car. 35 degrees doesn't seem to be a problem though.

If you're at an interstate rest area, the correct response to this problem is to start driving again. I think it's better to avoid cold parts of the country entirely, but if you do get stuck in such, you can always stay warm by driving. So, drive awhile, go to sleep, then wake up in a few hours hopefully before your car turns into an icebox again. You can either set your alarm for a reasonable time, or rely on your freezing nerves to wake you. One is probably more pleasant, but the other is more efficient for sleep.

Another trick is to sleep more during the day when it's warmer, and drive more at night when it's colder. However this can interfere with your "stopping points" for sightseeing on a journey, especially if you plan to meet someone at a particular time. A little forethought on how long you think you'll need to sleep and drive will overcome the problem however.

If you buy a DC extension cord, be advised that a long one (10 feet) is probably not going to work, because of the increased resistance in a DC line. DC moves electrons all the way down the wire, hence "direct" current. The reason AC "alternating" current is so interesting, is the electrons only move back and forth a little bit, they don't travel down the whole line. It's like attaching a string to a swing and using the string to move the swing. Tesla was a genius.

If you buy a short (coiled ~3 feet) DC extension cord from Walmart, be advised that it might be a POS. Mine seems to have gone kaput and I've only had it 2 months. Maybe it's fused and I can replace the fuse. I doubt I have the receipt anymore. Even when it works, it doesn't deliver enough power to charge my laptop battery. It will only maintain the battery, keeping it from losing any more juice.

If you forego your DC extension cord, and try to attach your laptop directly to the cigarette lighter, and you're lounging in the back seat, and you have a dog in the front seat, he's going to lie down on your cord and break your connection. After the 3rd time this happened I gave up and let him sleep. I'm on battery power now.

I have driven cross country from CT to CA and back again, six times. The most challenging by far were the solo motorcycle trips. I left Sacramento in 90 degree weather, and a few short hours later I was riding through a full blown blizzard in the mountains above Lake Tahoe. I had to put on every stitch of warm gear I had, and wrap all that in my rain suit. Thankfully that kept me warm and weather tight, but sliding around on two tires is quite a bit scarier than four.

But I made it down the other side of the mountain and back into warmer weather.

On the return trip to Cali a couple of months later, I rode into an even scarier situation. As I approached Lincoln, Nebraska, the weather began to turn frighteningly stormy. Hail stones the size of marbles started pelting my visor and fairing. The wind became dangerous, and the rain started falling horizontally. I made it to a rest area, and parked the bike next to a camper to protect it from the fierce winds. There was a short lull in the storm and the rain and hail stopped falling. I struggled to set up my tent in the wind and just as I made it inside and zipped myself in, the skies opened up and the rain fell harder than ever. Lightning flashed and thunder crashed almost immediately. The storm was right over my head. It was pretty terrifying.

The next morning, the nice Canadian couple in the camper invited me in for breakfast. They were very nice. The sun was shining once again and there was no sign of the dark skies from the day before. But once I got into the center of the city and stopped for gas, I realized just how lucky I had been…

There in the newspaper box, the morning paper's headline read: "Tornado Destroys Several Farms."

Going east I had ridden through a blizzard, but going west I rode through a tornado!

—- 

Yeah my original route last May was going to be through the various parts of the Mark Twain National Forest in Missouri.  Then Joslyn got destroyed and I said, screw dat!  I went much farther north, hoping to bypass tornado alley.  I guess that worked, I encountered no storms, but man was it hot.  Even South Dakota.  I kept thinking, if only I go farther north, it will eventually cool off.  Nope, doesn't work that way.  Even South Dakota is part of the Midwest weather system.  In fact I learned it's a big swamp, which ancient peoples traveled by boat.