The Rig

You don't have to stay in the cockpit when driving to the trailer, nor when hooking it up - nor when you arrive at the destination and have to turn it around to back it in. The game will tell you the instant your fail any optional objectives. Also, it will tell you when you have completed them - i.e. for this challenge when you reach the parking area at the end. The achievement will unlock when you complete and exit the challenge. Also you do not need to do all 3 optional objectives in the same run!