Honestly? Behind Walmart if you're in the midwest. NOBODY fucks with you there. 24 hour bathroom, cheap food for sale (canned shit, deli meats, granola bars) and there's usually security. The absolute best one I found was the north end walmart in Cheyenne, Wyoming. If anyone goes looking for it, there's a sweet concrete pad to the left of the walmart, it's actually behind Sams Club next door. But it's behind a huge growth of trees and it's fucking sweet. Just grab some cardboard to lay down & you're good to go.
Behind other strip malls are good too, you can grab pallets and make a little fort. Home Depot/Lowes etc are good to squat in those little garden sheds they have for sale. Just make sure to be out suuuuper early, because the staff start on shift at 5am and they might notice you coming out of them.
Light industrial areas are always great on weekends, because most small businesses are only open Monday-Friday. If you find a quiet area you could theoretically camp on a loading dock Friday night-Monday morning. Uhaul is a good one too, camp out in the back of the trailers or trucks in the yard. Just be careful; One buddy of mine got caught in the back of the truck box. The worker found him in there, slammed the back rolling door shut, fire up the truck and then started doing crazy donuts to fuck him up in the back. Opened it up, grabbed him and tossed him to the ground. Kicked him a few times, spat on him and told him to get the fuck off the property and never come back. So watch your ass and make sure you get out before sunrise.