The computation of dynamic slosh forces arising due to liquid motion within a partially filled tank is quite important in analyzing the directional behavior of tank trucks during various highway maneuvers. The most precise computation of liquid motion and the associated slosh forces involves solving complex non-linear fluid mechanics equations and is extremely cumbersome. The motion of liquid within a partially filled tank is herein investigated by representing the fluid slosh through an equivalent mechanical system using a pendulum analogy model. The model parameters are computed based on inviscid fluid flow conditions and the dynamic fluid slosh forces arising due to the dynamics of the vehicle during a given maneuver are computed using the equivalent mechanical system. The dynamic fluid slosh forces and moments are then integrated into the vehicle dynamics model to study the directional response characteristics of tank vehicles. The directional response characteristics of partially filled tank vehicles employing the equivalent mechanical system model are compared to those employing an available steady state fluid model, for various vehicle maneuvers. Simulations are performed to study the influence of various tank and vehicle design parameters on the dynamic behavior of tank trucks.