We always plan to stay in a campground, for safety reasons mostly, but also due to the truck noise issue as some mentioned. But, if we were dead tired or hit really bad weather or something... we might do it.
Years ago, my parents and I were taking a trip to Florida over Christmas in their Airstream. We had planned to stay at a certain campground, but we were in freezing rain, so we were nervous about the roads. We had stopped at a Sheetz (gas station chain) for gas, and dad asked for permission to stay there for the night. They agreed, and we pulled to the side of the lot and crawled in bed. Unfortunately, a truck pulled in next to us a while later, with his engine running, so we had that for the rest of the night.
I snapped this picture on my way to the truck in the morning.
He pretty much positioned his engine so as to be most annoying to us...