I use Flying J all the time. There are usually RV spots in front away from the trucks, commonly 8 to 10 spots. Not always very level.
I have seldom purchased anything in advance; I always eat breakfast there and gas up before I leave as thanks for having parked there. I have never observed anyone checking the parked RVs or questioning RVers.
The downsides of staying there are:
1. Traffic noise - the RV spots are usually by the highway.
2. Bright lights - I have draw shades over my ceiling vents.
3. Loudspeaker - "Shower number ### is now ready. Shower number ### is now ready" happens every 5 minutes all night.
I also stay at a lot of other truck stops. I can almost always find a place away from the trucks to park and I always ask permission first. Again, I make a point of gassing up and eating there if the restaurant appears suitable.
John W. Irwin
2018 Interstate GT, "Sabre-Dog V"