If you're just looking for a place to stop overnight en route
to somewhere else— which I assume you're asking about since you mentioned Walmart— many churches will allow you to park overnight in their parking lots.
And you can usually find churches in better neighborhoods than Walmarts, too.
If you're looking to boondock someplace scenic, and not just an overnight stop before hitting the road again, several Corps of Engineers lake projects have primitive campsites associate with the lake. Check out www.CorpsLakes.us - Corps Lakes Gateway
for more details.