I would stay in one in a heartbeat if I needed to. Of course I rather pay for a hook up rather than a propane cylinder, but sometimes it is necessary. While a WalMart parking lot is better than an interstate "Rest Area," an Interstate "Picnic Area" is better than either one. A Federal "primitive" campground is another consideration and they have dump stations.
Actually you can pretty much pull into anything that is designed to accommodate parking and get away with 8 hours. I saw a busy McDonalds on the interstate and they had a dirt lot down the street, I bet you could park there for 8 hours and no-one would bother you. If they did ask you to move tell them you are sorry and move it right away, do not undress, be prepared to open the door to greet any manager that might get after you. It would be nice to have a bag that shows you purchased something from them, that way they know you are a customer as well.
We stayed in a little city owned RV park with water and 50 amps, they had a single dump station as well. They took donations, I gave them $10 bucks cause that is what it was worth. I bet a lot of folks don't leave anything there.
There are plenty of places to get 4 to 8 hours of sleep when deadheading, You don't always need AC, I'm from South Texas and I promise you I have and still can sleep in 80 to 90 degree heat, granted that a little fan would work miracles in this situation. But a nap is a nap.
I have in the past covered up with a wet sheet and turn a fan on myself and it feels like air conditioning. It just depends on if you are saving up for supper or not. lol
"Dont be Ah-scared man," it is better to be forgiven than to get permission in many cases. BTW, I found that many people are willing to help out a fiver or a motor rig. I parked in a western wear place, disconnected the truck and had it serviced, I asked the manager before I did it and the guy was eager to please.
You might find that as long as WE are courteous, and try to repay the good deeds the better off WE are. BTW this is something all of us can do when we are forced into a WalMart parking lot; pick up a few of those items you know you need anyway, make your purchase and with your bag in your hand, go tell the manager how much you appreciate him/her and WalMart for helping RV'rs with temporary stopovers.