I have one of those insulated blankets that snaps on the front, and I am running fanless. I think yesterday was a high of -10. I run the winter front all winter long unless I am going on a long trip.
A couple weekends ago it was 40+ degrees out. (nice temp swing, eh?) Anyway, I forgot to take the winter front off as I headed up north. With the fan gone, it started to get warm quick, but I noticed it before it got to 210. Took the front off, and ran fanless no problem. I was not towing anything, just hauling some couches in the bed.
With the winter front and no fan, I can let it idle and it won't loose heat. In fact, it will actually go from 150 to 185\190, or whatever the t-stat is. When you are working outside and it is 0 degrees, the ability to maintain cab heat makes all the difference.