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 vehicle cameras and said trailer camera; wherein, with said vehicle cameras disposed at the vehicle and said at least one trailer camera disposed at the trailer towed by the vehicle, and responsive to processing of captured image data during forward or rearward motion of the vehicle and trailer, said image processor is operable to calibrate said trailer camera with said vehicle cameras; and wherein, responsive to processing of captured image data during forward or rearward motion of the vehicle and trailer, said image processor is operable to determine objects rearward of the trailer that are not in the fields of view of said vehicle cameras.