M1 tow barsWatch
Assuming the truck is towing rather than pushing, it exerts a force on the tow-bar in the direction of motion. The tow-bar exerts an equal and opposite force on the truck (Newton's third law of motion). The tow-bar then exerts the same force on the trailer in the direction of motion, and the trailer exerts an equal and opposite force on the tow-bar.
Only forces acting on the same object can cancel each other out and cause equilibrium. This is why in force diagrams it's often a good idea to draw separate diagrams if you want to show the forces acting on different objects.
*I believe - please correct me if wrong.