Re: No heat
At -15F or C, it is cold, and the engine will radiate heat from the block directly leaving less heat to transfer to the heater core. Some trucks can choose between fresh air and re-circulating air. If you can, put the heater on the re-circulate position. Maybe even putting it in A/C mode, on 'economy' with the temperature set as high as possible will help as you will recirculate the air, and the compressor will not kick in very much.
I have used pieces of cardboard in front of the radiator to partially block the airflow with some success.
If you have one, what does the temperature gauge indicate? If it is at the low end, you may need to replace the thermostat. They can fail to close completely, and leave a small gap between the movable and fixed pieces that wouldn't matter normally, but becomes noticeable at low temperatures. Get a GOOD brand, such as Stant or Motorcraft and you'll be more likely to have good results.