Autonomous guidance of agricultural vehicles is vital as mechanized farming production becomes more prevalent. It is crucial that tractor-trailers are guided with accuracy in both lateral and longitudinal directions, whilst being affected by large disturbance forces, or slips, owing to uncertain and undulating terrain. Previous research has been concentrated on trajectory control that provides longitudinal and lateral accuracy if the vehicle moves without sliding and/or the trailer is passive. In this paper, we extend current results by addressing the problem of robust trajectory tracking along straight and circular paths of a tractors with steerable trailers. We develop a controller that is a robust combination of a backstepping and nonlinear PI control. For vehicles subjected to slip, the proposed controller makes the lateral deviations and the orientation errors of the tractor and trailer converge to a neighborhood near the origin. Simulation results are presented to illustrate that the suggested controller ensures precise trajectory tracking in the presence of slip.