Great job on getting a early start to the winter camping season. We are still waiting for winter to arrive in the PNW....though it might be happening right now.
I tend to agree with putting them inside. But, in an effort to come up with a different solution, you could try; in the back of your truck, inside an ice chest, that is wrapped up in blankets. You could even toss a couple heat packs in the ice chest if you wanted to be sure....but I think you would be OK without them for just overnight.
But now that you know the weak spot in your plumbing (where it freezes), I would be inclined to solve that and not worry about separate containers of water (until you go somewhere it is colder). Adding insulation in that area perhaps? Or box it in and pump some cabin air down there? Or recirculate the water so the pump cycles occasionally? Or, maybe a really low power heat tape that you can turn on only occasionally and is covered by a good layer of insulation. Whatever you do, I really doubt it will take much.
