We've boondocked overnight in a US BLM area in Quartzsite, Arizona. Another time we had some RV trouble and stayed overnight in a park's parking lot in Medocino, CA. Felt safe both times.
Warning... Completely off topic:
Funny aside on the Mendocino thing. We pulled off the freeway when a passerby pointed to our 5th wheel in a way that suggested we had a problem. We followed him off the freeway and pulled over on the exit. He suggested we pull into the park that was within visual range to sort it out there, and wished us well.
Once in the parking lot a couple of people realized we were having problems and came over to see if they could help. One (who was experienced with horse trailers and RVs) immediately diagnosed our problem and recommended a nearby business to sort it out. She called the guy for us. He was on another emergency though and couldn't get to us before morning. By this time it was early dusk.
Posted signs said the park was closed overnight, but she said if we explained (and showed our problem) to the local police who'd surely come to check, they'd probably let us remain. Which they did. About ten minutes after the cop locked the park gates the woman called my cell phone to ask if we were ok, and offered to let us come to their house and stay in their RV overnight. We declined but thanked her kindly.
Shortly after someone else with a flashlight showed up. Turned out some of the mansions across the street had 24 hour armed security. This guy patrolled that handful o homes all night, making sure all was well. He promised to keep an eye on us, too.
Early the following morning the repairman came, sorted us out for far less than we'd expected to pay, and we were on our way.
My point, and I do have one, is that kind, caring people are everywhere. Sometimes you don't even have to look for them - they just show up when you need them.
@AnnieA, I hope when you're boondocking in Walmart parking lots and other freebie places that you run into the same friendly, helpful people we're encountered.