This topic always draws a large response.
Diesel engines do indeed need a cool down and warm up periods but 5 minutes of slow driving off the highway to the campground is sufficient to do that job and a 5 minute idle to bring up the air and slow drive out of the campground again is adequate and the "healthy" thing to do. By all means watch the temp gauge to be sure it has reached manufacturers suggested operating range.
However a slow idle under 1000 rpm is not wise ever except at 1st start up (for 1 minute) then it should be kicked up to 1000 rpm (with the use of the cruise control when the brake is set & in neutral). That is the manufacturers recommendation not my opinion. Even stopped at a traffic signal or waiting for a construction delay...(set brake, put in neutral and kick up to 1000)
Camp Frieghtliner is a wonderful course and teaches proper engine care. It was very helpful for us a newbies way back when. Each manufacturer may have a few minor differences in suggested operation so it is well worth every driver's time to learn all they can learn.
Yet even with all this knowledge around the RVing community we still see big rigs sit at slow idle while folks are out fusing with hoses, hooking up toads, checking the tires, chatting with the neighbors...etc. In fact last evening we went out to eat at our local State Park. Rigs were pulling in for the weekend, no doubt many of them enroute to Sturgis for the big rally...and low and behold a big Discovery sat out front of the lodge's office at slow idle for 20 minutes!