Safety while stopping overnight to get some sleep

We've done this occasionally for 20+ years. If you're really concerned, pick places that other RVers are parked. However, we've never had a reason to be concerned. Truck stops are noisy and we don't like them for that reason but I would consider them safe. Rest Areas - we wouldn't exit our RV to use the bathroom during the night but otherwise, o.k. If you have a smaller RV and find a Cracker Barrel, that would be good. Most of their parking lots are not conducive for big RVs. We really like casinos. WalMarts are just fine. There are plenty of other folks around and there are usually patrols that drive by all night long. I'd advise you to let the store manager know that you'll be staying - ask for permission and they've never refused. The parking lots also have security cameras. We've never heard of problems in our 16 years of full-timing and talking with other full-timers. As others have said, just use common sense in the surrounding area and you'll be fine.

Safety for DW and myself and maybe a grandkid or two is well worth the $30-35 or so for a private RV park. Doesn't have to be fancy.

Avoid heavily-populated areas. More people= more thieves, and they're too lazy to venture too far to rip you off.

My favorite, other than Casinos, is small-town truck stops, or truck stops along freeways. Most of them don't smell too bad of urine.

I think a campground would be safest.

On my last cross-country trip I stayed in a few parking lots. The only trouble I had was the very first night when I pulled into a rest stop. I found a parking spot and had got out of my seat and was walking to the back when there was a knock at the door. It was several obvious transients who wanted a ride. They were a little demanding, but went away when I made it clear I was not giving them a ride. So we pulled out of there and went down to the next town and stayed in a Walmart parking lot.

If you do a Google search on rest stop crime you'll see some rest stops tend to attract some unsavory characters.

Use common sense, I have stopped at some campgrounds and was spooked and left quickly without staying. Some advice is: never open your door when stopped. Keep locks on doors. Don't venture out to the rest area bathroom late at night. Park in lit areas and not in darkness. If you feel the least bit uneasy it's time to move on.

Can't know until ya get there.

Side note:

When overnighting in such informal settings, I'm prepared at every moment to move along if necessary. I often don't even change into my jammies; and NEVER have my customary evening beer.

Ya never know what might come up, and being prepared to just throw on one's shoes and git seems prudent to me.

No Cracker Barrels and only some Walmarts are open 24/7, so limited or no security there. Crooks don't like witnesses. Truck Stops and Rest areas are noisy 24/7 and may or may not have security. If they don't, there may be lot lizards stopping by to see if you want company.
OTOH, CGs and RV parks usually have one way in and out and tend to write down the names and license numbers when registering someone. Not a crook's first choice.

For Walmart camping, I avoid cities and large towns. In smallerl towns, I park well way from any fast-food restaurant which could be a place where local teens gather in large numbers. (Not that they would necessarily be dangerous, but teens can make a lot of noise.)

you need to be aware of your surroundings and get a FEEL for the area... basically...X5 on what they all have said...