Ah, the art of car bumming ...Good topic! Done it on many a trip. Here are a few pointers:
First, you need the right vehicle. Used to have a big van, but it costs too much to drive. A Subaru leaves you sleeping on too high a platform -- up in public view. I found that a Rav4 was just long enough for me to stretch out in, with the front seat pushed all the way forward, and allows me to sleep down, out of sight of pedestrians, and at a mileage I can afford.
Then you have to set up your vehicle. I put down a 3" foam pad, a warm sleeping ang, and a down comforter over the top. Cozy! I have a thin tarp that I can rig with ties I've attached so that it hangs from the ceiling and covers the windows. So I am in a tent in the car. This is especially useful when sleeping in places that you are not suppsed to use for car camping, such as National Parks, ritzy hotel parking lots, and the like. I have had security try their flashlights on me, and even bang on the door and ask if anyone is inside, but they always leave after a minute or two. You DO have to keep a hand over the dog's muzzle on these occasions, however!
Next is the all-important location. Parking next to the lift tower is often a good choice before powder days, so long as you are not impeding the snowplow. As a matter of fact, I often sleep in a ski area parking lot that will get plowed. The plow WILL wake you up, and you can quickly move to an area that they have already plowed and continue the night's sleep. A dark corner of the ritziest hotel around is a good choice. If you have packed a decent pair of pants, you should be able to wander in with a washrag and a towel under your coat, improvise a suitable sponge bath in a toilet stall, and spend the evening reading in the lounge before retiring to your cozy nest in the parking lot.
Or use up a couple dollars on an athletic club shower, pool, lounge for the evening.
Then there is the art of leveraging a hostel. I quite often spend a night in a hostel when I get to a town. I get good local conditions information, a good shower, and a community kitchen to prepare a couple days' food. Then, the next night, already being a familiar face, I find I can usually access the kitchen again for a meal, and maybe even cop a shower. Don't try to sleep in the hostel parking lot, though; too many folks try that one, and you don't want to be identified with these dirt bags!