Caterpillar recommends leaving the engine rung for about 3 minutes after running at speed on the highway. As mentioned, this is so the turbo cools down and doesn't fry the oil in the turbo bearings. By the time you slow down for the exit and find your way to the pump, you will be way past of the required cool-down, and can shut off the minute you pull up to the pump.
By the same token, there is no need for an extended warm-up when starting out with a cold engine. We run at idle out of the campground, and keep the speed below 55 MPH until the transmission temperature gage starts moving before we go to our cruising speed.
