According to this article, it may not be that the battery is drained, per se, but rather that it has lost quite a bit of its capacity.
The article states a battery will lose:
- 35% reduction @ 32°F
- 60% reduction @ 0°F
Plus, it takes a large increase of amperage to turn the engine at the lower temperatures. This is the double whammy. Just when you need more power to turn the engine over, there's less capacity for it to happen. Plus, it will most likely take you more cranking time for the engine to catch and start. Just the nature of the beast.
Your battery could just be suffering from this. Realize, a normal battery's life is between 3-5 years. If yours is closer to the 5 than it is to being brand new, it's useful life is greatly truncated. The cold only makes it worse. Over time the battery will lose capacity as well. The 35-60% decrease they talk about in the article is the amount subtracted from the decreased amount of the older battery.
As told at the Battery University, automotive batteries will usually continue to have decent Cold Cranking Amps (CCA) over time, but their reserve capacity drops significantly as they get older. This means, like in your truck, if it's a bit old, you might have used up your reserve capacity quickly, especially considering the cold weather. Like I said, double whammy.