It is a diesel. They are hard to start. You need good fresh glow plugs and a new fuse. Change that stuff it easy. remove the fuel lines and change out the glow plugs, it is as hard as changing spark plugs. Also glow plug the car for ten seconds before starting it. (turn the key to the pre glow position and count to ten...the glow plugs glow longer than the interior light indicates) It makes a big difference and saves the life of the starter which is not easy to change and is expensive. The glow plugs are 7 bucks each, the fuse is 2 bucks, get new fuel filters if you havent this year as you will be priming the fuel system after removing the fuel lines and wahla you have winterized you heavy beast.
I may be crazy but I might change my plugs every year in October to prepare for the cold winters in chicago. 250000 on the monster I want to give it the best chance to start at 10 below that I can. I also run Delvac one 5w40.
Fix this problem soon and replace all the plugs