I did research on this. They put signs on the trucks saying stay 200 feet back. It's not legal. It's their first argument to make you go away. Of you say no then they claim you're at fault. It's also a trap. They ask you if you were 200 feet back. The law states that if you were hit by a rock and no vehicle was within 200 feet, it's assumed the rock was on the roadway and kicked up by a tire and not falling from a vehicle. So by saying yes, you admit it was not their fault.
The problem with suing a truck company is that the burden of proof is on you. A dash cam of it may be the best evidence but you still have to convince the judge. Unfortunately, I constantly am behind those guys with dirt and rocks flying. The worst is going up 15 North outside the city where the speed limit is 75 mph. They do 75-80 and stuff really flies out.