There are a number of choices... Wife and I often overnight at a Flying-J truck stop when we are on the road. (Though we like to hit a proper camping ground from time to time.. Shower,,, Shave, That kind of stuff)
Many folks "Wall-Mart" in this context Wal-mart is generic for any BIG parking lot, Shopping malls, Sam's Club, Costco, Meijer's. it has to be big and have lots of extra space and no signs that say "NO OVERNIGHT PARKING" Church lots are another place in some areas.
If you are a member Moose, Eagles, VFW and many other faternal orginations have parking in many cities
If you are a member of a campground and thus eligable for Coast 2 Coast membership, there are hundreds of C2C campgrounds. Many guarentee you one night parking for 8 bucks reservation or not (you may be stuck off in overflow, or even the sales parking lot) just one night, no more w/o a reservation. (I have to research that rule)
There are lots of options other than rest areas... Many rest areas say "no overnight parking" by the way or NO camping.
Rule, If you are at a Flying-J, Pilot, Loves or other truck stop, Or a Wal-Mart, (Or other store/mall parking lot) do not drop jacks, do not Set out "Stuff" (Grills, lawn chairs, mats and the like, Keep the awning rolled up, and unless you have a rig like mine with a transverse bed where you have to put out the slides to sleep.. Don't put slides out (I put out just one of my 3 slides, Often the wardrobe) Unless there is a specified RV parking spot (Flying-J has those) park in the most unused section of the lot (Out of the way) do not park right in front of the store. And do shop the host.. Of course at a Flying-J you will often drive till a late dinner, dine at the eatery, sleep, breakfast at the eatery, fuel the rig at the pumps. (This is why they want you parking there) At other stores, try to give them business if you park there.
I avoid Wal-mart because I do not like giving them business... They have given too many people "THE BUSINESS" If you know what I mean, very unfair to labor, most sued company in history when it comes to labor issues, and they loose more than any other company in history too. I don't like their advertising, misleading decor and several other things. So I try not to stop, or shop there. Though I have, from time to time, found they had what I needed and nobody else did, or I parked along the edge in back of oh, say, Sammy's Pizza, for lunch (At the pizza place) Note, name of restaurant is fictional