1. A driver assist system for a vehicle, said driver assist system comprising: a plurality of vehicle cameras configured to be disposed at a vehicle and having respective fields of view exterior of the vehicle; at least one trailer camera configured to be disposed at a trailer towed by the vehicle; an image processor operable to process image data captured by said plurality of vehicle cameras and by said at least one trailer camera; wherein, with said plurality of vehicle cameras disposed at the vehicle and with said at least one trailer camera disposed at the trailer towed by the vehicle, and responsive to processing of image data captured by said plurality of vehicle cameras and by said at least one trailer camera during forward or rearward motion of the vehicle towing the trailer, said driver assist system calibrates said at least one trailer camera to be in accordance with calibration of said plurality of vehicle cameras; wherein, responsive to processing of image data captured by said plurality of vehicle cameras and by said at least one trailer camera during forward or rearward motion of the vehicle towing the trailer, said driver assist system determines objects rearward of the trailer that are present in the field of view of said at least one trailer camera and not present in the fields of view of said plurality of vehicle cameras; a display screen disposed in the vehicle for viewing by a driver of the vehicle, and wherein said display screen displays video images derived from image data captured by at least one camera selected from the group consisting of said plurality of vehicle cameras and said at least one trailer camera; wherein said driver assist system generates graphic overlays at said display screen overlaying the displayed video images, the graphic overlays indicating a projected path of the vehicle towing the trailer for maneuvering the vehicle and trailer towards a target parking space; and wherein, during a reversing maneuver, and responsive to a determination of an object rearward of the trailer, said driver assist system adjusts the projected path to avoid the object.