Water is so important. Water is the giver of all life, and your water is going to freeze.
If it’s warmer than ten above and you run your van to the point of toasty warmness 2 – 4 times a day and keep your water up off the floor it might stay liquid. As soon as it drops below ten degrees, though, your warming a losing water jug. At ten below and colder, just give up. It’s going to freeze. Accept it and move on.
Interestingly, glass seems to insulate water for longer than plastic. Also, a glass jar of water can be set on the defroster to get hot or thaw out with no worries of toxic chemicals. I’ve been using canning jars, but I’m looking to upgrade to a wine bottle. If your very patient and resourceful, you can even melt snow in your jars on your defroster to make water, but this takes a very very long time and you might as well just boil water from a river or drive to town and fill up your water container someplace.
As for food… think of this as an opportunity to buy all that frozen stuff that you couldn’t eat all summer. Sausage, bacon, and all varieties of frozen meat. You can even eat yogurt and veggies and stuff that needs to not freeze – just keep it under your feather bed down by your feet and try to keep your dog from stepping on the fragile stuff. When canning for the winter, remember to leave plenty of headspace for your stuff to expand as it freezes. To use the stuff, just set it on the defroster until it thaws out enough to get it out of the jar. If your potatoes or other starchy veggies freeze you can still use them, just don’t unfreeze them until you cook them.