You have two general types of locations: the large travel plazas on interstates and rest areas on two-lane secondary highways. The latter are becoming things of the past, as states don't have funding and personnel to maintain them. Unless you're really in some form of need, I just wouldn't stop at one. Typically, they're not well maintained and tend to be hangouts for the local criminal element. I definitely wouldn't sleep in my vehicle in those locations.
The larger travel plazas/rest areas tend to be better maintained. These will typically feature a gas station with a convenience store, restaurant and truck park. The downside here is they tend to feature all of the city type crime we're familiar with. When I worked the Kansas Turnpike this is what we had. It was like having a small town every 30/40 miles.
When frequenting these establishments AVOID THE TRUCK PARK. This is where the majority of the crime takes place: vandalism, rape, drug dealing, prostitution, etc. If you're thinking of parking your RV or vehicle in there to get some sleep don't do it. Unless you want pushers offering you dope, or lot lizards selling blow jobs knocking on your door every hour. Consider the truck park shitbag central, or the equivalent of Mos Eisley. A wretched hive of scum and villainy.
If you stop with your family members: there's strength in the pack, stay together. Don't let your children even go to the rest room alone and men, always use a stall instead of a urinal. These places are very busy during peak travel season and it's very easy to lose your kids, permanently. In one year we had three kidnapping attempts at one such area. Two young women were nearly abducted by a human trafficking ring and one estranged father attempted to kidnap his daughter from his ex-wife. Fortunately, none of these were successful. So, keep everyone together and minimize the distance you get from your vehicle.
If you're gasing up, everyone stays in the vehicle until you're finished, then you move the vehicle to the store front and everyone goes in together. Up thread, someone mentioned going in and checking things out first, while leaving the wife and child in the car. In reality, your wife was in more danger sitting in the car than you were going in. Stay together. Also, don't leave your vehicle unlocked and don't leave it running. You'd be agog at how often these two things occur.
I don't want to give everyone a case of paranoia about using such facilities. Many of them are quite nice and are a source of state pride. In the vast majority of situations you'll make a stop without incident. It's not Mad Max World and there's no guarantee you'll meet the Toe Cutter at the slushy machine. Afterall, sometimes you just gotta go and maybe you just need some Beef Jerky and a Kansas key chain. Just keep your head on a swivel, stay together and if your inner voice is telling you it seems hinky, listen.
Oh, and pack a gun. 😉