I do not "overnight" at truck stops, WallMarts, or interstate rest areas. I overnight at KOA campgrounds, so I'm not among one of the offenders.
But reading your post makes me wonder, and the next time I'm at a truck stop, I'm going to look....
Is it posted anywhere that the parking spots are designated for big-rig trucks only? Are there any rules by the business establishment that RV's cannot park where the big-rigs park? Considering truck stops are privately owned (they are not government organizations are they), they each make their own rules and maintain their own parking lots at their own expense. So if they do not have any such rule about parking, I think the parking spot is fair game ... first come, first serve!
Now ... roadside (interstate) rest areas, ALWAYS are posed automobile parking one direction and Trucks with trailers/RV's in the other direction. So us RVers are not wrong for parking there.
What does get my goat, is when automobiles park at Cracker Barrel in the spots that ARE clearly marked RV only. So, I understand how you feel, not getting a spot, but if there's not rule a or guideline, sorry.... it's "first come, first serve!"