Some things to think about...
The trucks are on wheels that turn?
If so then the friction between the wheels and the ground won't resist the movement like that. The dominant retarding force will come from friction in the axles... this will depend on the speed so for a constant applied force the tractor+trucks accelerate to a maximum speed which may be less than the maximum speed of the tractor by itself.
The maximum force the tractor can apply, though, will be it's weight multiplied by the coefficient of static friction between the tires and the concrete. It will have to oppose it's own losses as well as those in the trucks it pushes - when the applied force is equal to the total retarding forces, the trucks + tractor move at constant speed. While the applied force is higher - the whole will accelerate.
Does the tractor push the trucks through their center if mass?
If not, then the tractor is applying a torque to the trucks. Not all the force will go into horizontal motion.