Leave your car in ready mode with the AC on. Load the trans against the park brake before selecting park then sleep in airconditioned/heated comfort. Yes I have done it on a 30 hour 3000km drive more than once. Why not stop at a cheap motel? Because a lot aren't accepting guests at 1AM when you want to go to sleep and showers only slow you down. 5 hours sleep is plenty.
The Yahoo group "WalmartRVing" is dedicated to mapping which Walmarts accept overnight RVs and those that don't. The corporate policy is to allow it where it is legal and doesn't interfere with their business. The proper protocol is to check with the service desk for permission to stay overnight. This is worthwhile even if there are RVs already parked, and/or there are signs prohibiting overnight parking. In some places there are unenforced laws that prohibit such parking, so they are signed as such. But if the manager gives permission, nobody gets upset. In other places the signs are there to keep undesirables away, so clean up and look responsible before approaching the service desk. There is usually a preferred area of the lot to use, so be sure to ask about that as well. It would only be fair to do some shopping at the store if they let you use their parking lot.
As goofy as it sounds, walmart actually pushes for people to stop and sleep there. Because they know you'll probably come in and buy something, plus they have the lit space. We used to stop in our suburbans and sleep on long road trips. Worked well, except for the loud 5am lot sweeper.
To the astute predator, the Prius camper is a morsel indeed...especially when a young female is a bonus. Learn to use and carry a firearm. Sadly, many victims and their families wish they had...
I'll try to clear your confusion: Apparently there is plenty of concern here about picking safe places to snooze. Bad guys make their living by profiling and selecting their victims.The Prius driver is more likely to be perceived as passive and law abiding than say the sleeper in the mud-spattered crewcab with a gun rack in the back window. Hulk Hogan nor Ray Lewis are not likely to be sleeping in a Prius. I'm the exception. At 75 I'm fit and strong but certainly not a tough guy, so my 357 mag is always next to my pillow. I maintain situational awareness and good judgment while not worrying as much as some. Sometimes my 85lb cur is curled up next to me as well. My professional driving career involved many hrs on the road and I've decided that rest stations often attract miscreants doing drug deals or other unsavory pursuits. They are by definition in more remote highway locations, are unattended, and offer quick escape. Better to pick the travel centers such as Loves, Flying J, Pilot, etc. They are staffed, better lit, and the clientele purposeful, and I've never been awakened by the sweeper. They have showers, if desired, and they more regularly refill the coffee. Oh, and here is the obligatory riddle: Why are police stations safe?.................because there are people with guns inside.