Still the trucks fault, no matter what. It is his responsibility to know where his truck is and to keep it out of traffic. You cross the center line and hit someone, it is your fault. And more than likely he missed the "No Trucks" sign at the beginning of the highway.Not necessarily if he drove into the trailer and the truck was pulling reasonable speeds even going into the oncoming lane. It's impossible for that truck and trailer not to end up in the oncoming lane.
But yeah, more than likely the truck wasn't watching his mirrors.