Supposedly, fuel companies put additives in diesel fuel to keep it from gelling in temperatures down to about -20 to -30F. However, Ive read that gelling can occur if the fuel sits in the tank at 5 to 10 degrees for an extended period of time, (especially with Biodiesel, the more "Bio" the greater the gelling), and can clog the fuel filter
However, I always make sure when fuelling the truck that I put diesel in both tanks. The Transfer Flow system actually pumps fuel from the secondary tank to the main tank automatically so fuel does not just sit in the secondary tank for an extended period of time.
If I did not have the transfer flow system, I would make sure I used the fuel in the secondary tank on a regular basis. I would not let it sit in the tank for an extended period of time as it would tend to collect water (another issue with diesel fuel) which is actually a worse problem for your diesel engine. Water in the fuel gets into the cylinders and, under the high compression diesel engines operate at, forms tiny steam bubbles which can actually pit the cylinder walls.
However, I believe most diesel pickups made also have water collection systems to prevent that from happening.
All in all, most any diesel truck you buy can normally be run on a day to day basis without much more regard that you would pay to a gas engine truck.