Walmart encourages RVs to stay in the lots of their 24 hour stores, as long as it's not against local law. You will get rousted in Gainesville, Florida, for example, regardless of whether you have permision from the Walmart manager or not. They have posted their lots to this effect. You used to be able to buy a camping Atlas from Walmart that had every Walmart store marked on the maps. There are some unwritten rules that all RV folks seem to understand intuitively; they use the lots as a point of convenience to get a night's sleep; you won't see awnings opened or lawn chairs put outside, for example. Most municpalities that have rules about overnight parking allow people to stay in a location like Walmarts if it is private property and they have permission.
Interstate rest areas usually have posted signs saying that overnight parking is not permitted; almost every area, however, has sleepy motorists sleeping in their vehicles, and sometimes they oversleep to an all-night status. These rules are usually not enforced unless they are flaunted. Staying in an RV in the big-rig parking area where hundreds of truckers are in their sleepers is pretty hard to regulate.
Travel plazas like Flying J often set aside specific areas for RVs to park, and usually, no one checks to see how long they've been there. Once again, even in a city that has restrictions, it's pretty tough to enforce them if there are dozens, if not hundreds, of truckers asleep. Most cops are smart enought to know that it's better to have a sleeping driver than a sleepy driver, and they look the other way, again, unless there is some flagrant misuse of the privilege.
I've also had great success going into the nether regions of hotel parking lots, where truckers and RVs park when using the hotel. I just tuck myself into the middle of the legitimate RVs and stay discreet.
I've also gotten away with discreetly sleeping in the RV in the parking lots of convention centers where trade shows are being exhibited. There are so many trucks and RVs belonging to exhibitors, it's hard for authorities to pick out who belongs there, or not. I even mixed in with the campers from a traveling carnival at a fairgrounds one time.
But, almost every municipality has rules against just pulling up anywhere and setting up "camp". For me, it's almost become a "game" in which I see what I can get away with. One time, Betsy had a continuing education seminar (she's a physical therapist) in a hotel in downtown Orlando. We took the old Winnebago, and expected to hide in a corner of the hotel parking lot. When we got there, it turned out to be a parking garage, and we were too high to fit inside. I looked around; we were right across the street from the Chamber of Commerce. It was Friday night and they were closed for the weekend. So, I backed into one of their parking spots, pulled all my blinds and kep the lights low, and stayed there for the night. The coach looked to be unoccupied, so no one bothered us.
Another time, she had a seminar at a rehab clinic; we camped for two nights in the center's parking lot and no one questioned us. So far, I haven't been rousted, but several of my friends have been. We once attended an autocross (a sports car racing event in which one car at a time races against the clock) at Opryland in Nashville. The course was set up at one end of their parking lot. They gave us specific rules that no one was to stay overnight, and they would be patrolling. Well, about 12 of us decided to stay, anyway. There were about 30 RV's parked in the "paddock" area; many of them were in the hotel. During the night, about 6 of the RV's were rousted and told to move; the rest were ignored. It pays to have light-proof window coverings!
The bottom line, is, if we intend to stay somewhere more than one or 2 nights, and if we want to relax outside the RV during the day, we stay in some sort of recognized facility. We aim for national, state and local parks with RV spots, but we'll stay at a KOA or similar if there is nowhere else.