“Safest places” would be RV parks or campgrounds with full time on site management. If you are looking for free overnight stays then many people stop at parking lots of Walmart, Crackerbarrel, other businesses with large uncrowded lots, highway rest areas where allowed, etc. Safe then depends a lot on the environment of the local area."
We have been using the places listed above for 20 years and have never felt threatened. We've had a couple of strangers knock on our door and looking for a hand-out, but took no for a answer and left. We have parked in a couple of walmart parking lots in city areas that were a bit seedy, but had no problems.
Just be sure to ask the manager of walmart, etc. if it's ok. Some cities have ordinances against boondock parking and I think the cops think it is humorous to wait until 2 a.m. to bang on your door and make you move on.