I've camped quite a bit in freezing temps and I'll second what go4aryd said with the disposable hand warmers. This works great for short camping trips of a day or two. Just stick a hand warmer under the container before you go to bed at night and if you have a wool blanket wrap that around the container. Even better if you can get a few layers of wool blanket under the container and the hand warmer.
That will keep it from freezing if the outside temps drop as low as 10F outside your truck during the night. If you get up in the morning and ice has formed near the top of the container, just shake it real good and the ice will break up enough that you can pour water out. The hand warmer system works great as long as the temps are above about 40F sometime during the day and you can get your container out into the sun.
If you are in a prolonged cold exposure setting where days and nights never go above 32F, then you could go with a 12v heater of some sort which I've never done.
One thing that does work well for long camping trips below freezing is to buy an old down filled mummy sleeping bag off of craigslist. Cut the mummy bag in half and only keep the lower (foot) portion. Stitch it back up with some thread so the down doesn't spill out. Basically you want to end up with the mummy bag about 3 feet long and it will look like a kitchen garbage bag sized bag.
Next, take your dutch oven and put it in the bottom of the mummy bag after you've eaten dinner and cleaned up. From your campfire, heat up a couple of rocks and put them in the dutch oven and put the dutch oven lid on.
Next, take a couple of tree branches that are about 4" long and about 1" in diameter and place them on either side of the dutch oven's lid handle. This creates a flat platform to set your reliance container on.
Next, put your water container on top of the dutch oven and tie up the mummy bag like a garbage bag. The dutch oven will keep the plastic container from direct contact with the hot rocks. You'll quickly figure out how often you need to throw a newly heated rock into the dutch oven to keep it above 32F. It shouldn't be more than once a day that you need to throw a new hot rock inside and you don't even need big rocks. A couple of fist sized rocks work well.
The nice thing about this solution is that you don't have to store your water in the truck, you can keep it outside next to your kitchen area and you are repurposing your dutch oven into double duty which is always good. It also doesn't use any batteries and the mummy bag will scrunch up to the size of a softball making the whole system very compact. When you need to use your dutch oven for cooking, just wipe the ashes out with a paper towel and off you go.
A couple of tips - you don't need to use a glowing red hot 1,000F hot rock. That will be too hot and melt your mummy bag. Just heat up a rock for about 20 minutes in the fire and it will be plenty - or just keep 4 or 5 fist sized rocks at the edge of your coals at all times. No need to keep the rocks right in the hottest part of your fire. Also never use wet river rocks in your campfire. They will explode because of the expanding steam inside.
Happy Winter Camping!
