There would certainly be some upfront cost to making tractor-trailers autonomous but they would save an expensive employee per truck, the trucks would arrive faster and more reliably, and they would save in fuel costs (optimal driving).
They would need to setup a way for the trucks to refuel, just an attendant at the truck-stop that would be paid a small amount for refueling them, or maybe a new kind of pump that would automatically fuel the trucks when they pulled up to them.
They would need to setup a way for the trucks to refuel, just an attendant at the truck-stop that would be paid a small amount for refueling them, or maybe a new kind of pump that would automatically fuel the trucks when they pulled up to them.