But where?

I started out overnighting in Walmart parking lots. You can find out online which ones allow overnight parking and which don't, or it's easy to tell by checking for tractor-trailers and RVs in the lot.

Lately, though, I prefer to camp on National Forest land, State Forest land, or other campgrounds. It's much nicer to wake up in the woods than in a strip mall parking lot.

 

I go to biker rallies. I trailer the bikes and we gather at bonfires at campgrounds in the mountains of NC, TN. My friends all stay in tents or cabins. I cant sleep if it's humid or hot. Call me spoiled. But I love camping w friends. Reminds me of childhood. The Prius is the best combination.

 

I've caught a battle nap in the truck parking section of rest areas....and in Walmart parking lots where the R/Vs are parked.
The only problem with Wally parking lots is that some LEO might think you're sleeping off a drunk (out of state tags and parking placement help here.

Generally speaking though, I'm a little leary about sleeping in a car unless I can lay flat.
BAD habit to get into if you're driving tired.

I do not worry about bi-pedal predators since I'm protected by age, gender, and shot shells.
Quadrupeds usually cannot get into the car....but I will always give them the benefit of a horn honk before working my way up the ladder of force.
(Show, Shout, Shove, Shoot...)

 

I've napped multiple times in rest stops. Most of them have an 8 hour limit but that's plenty of time if you're just passing through to another destination. I carry a twin sized inflatable mattress on those long trips. Fold down the back seats and it fits between the wheel wells. I just move luggage, ice chests, etc. to the sides or to the front seats and it's pretty comfortable. I even get to use my CPAP machine by either leaving the car on or taking along a portable power supply/air compressor.

I don't know about the rest of the country but many WalMarts in southern California have discontinued overnight stays in the parking lots. I guess too many homeless people, problems and liability issues. My golfing buddy has been woken up in multiple cities by the police or security guards and told to move his camper because of the new policies.