According to Newton's Second Law, the net Force acting on an object is given by

Fnet = MA

The acceleration is given by the ratio of the change in an object's velocity and the time it takes to change

a = (Vf - Vi)/T

If the vehicles are traveling at constant speeds, then their change in velocity is zero (0).

If the change in velocity is zero, then the acceleration is zero (0).

This gives that Fnet = M(0) = 0

Therefore, the net forces on the car and the truck is zero.