Safest are the regular truck stops. They have security to protect the drivers' rigs from theft and to keep out the hookers who otherwise would be waking the drivers up during the night.
Next safest are the rest areas along the major highways. There are usually other people stopped there so you will not be the only ones there. Law enforcement also will check these places along their route.
24 hours restaurants and gas stations will also have a lot of traffic and be noisier but also relatively safe.
Least safe is any parking lot in town where you are the only RV or vehicle in the lot. But even then the odds of someone breaking into your RV are extremely remote and not worth worrying about.
The only place that is truly dangerous is parked beside the road where you take the risk of a driver not realizing you are stopped and plowing into you at 70 MPH. That is how most law enforcement officers are injured on the job - while stopped beside the highway in their cars.
