When cold, the diesel does not combust fully. Slow idle for a long time tends to wash the cylinder walls of oil. Will you destroy the engine by that idling? Not very fast.
However, the best way to warm up depends on temps. Most of the time we are using as RVs, the temps are above freezing. Above freezing, start the engine, and let slow idle for about a minute. Then kick it up to about 1000 rpm for 3-5 minutes. Then take off.
Do not use more than about 1/2 throttle at first. Usually we are leaving a camping area, streets, etc. So, we are going slow anyway. If you happen to be at a rest area on the Interstate, just slowly accelerate in the merge lane. Gradually getting up to 55 or so. Keep an eye on your water temp gauge. Once it starts climbing, you are good to go.
The faster you can get it up to temp, without pulling hard when really cold, is the best. As it gets colder, then you need to lengthen these times a little, but same process. Just do not pull it hard until the temp gauge starts up, and it is good.
From a long haul trucker with over 2 million miles. Owning my own trucking company.