Nowadays, we always stay at a campground. We used to stop at
Walmart, and while it never felt unsafe, it was very noisy with the delivery trucks coming in and out all night long. We stopped overnight at a rest area out east just once, and even though it was a designated RV area, it didn't feel safe, so I was awake the whole night.
We used to stop at truck stops quite often until a scary experience in Chattanooga (off I-75). Similar to some other posters, we were eating lunch in our travel trailer, middle of the afternoon. Someone came knocking on our door and tried to lure dh out by telling him somebody was giving away free lottery tickets. What really unsettled me was that just 10 minutes before, dh was inside the restaurant getting food, while my young dd and I were out walking our dogs near our trailer in the back, where all of the trucks were parked. I thanked my lucky stars that nothing bad happened that day, and since then the only time we get out at a truck stop is to pump gas and stretch our legs in the front of the building, near the road.
I don't mind paying just to sleep---in fact, I'm really picky about even our overnighting campgrounds, so I sometimes pay quite a bit just to sleep. After that horrible experience in TN, it's worth the peace of mind for me.