I'm a complete noob at this and live in the deep south, so take this with a grain of salt, but solutions I've seen use aquarium water heaters, electric pipe wraps, etc. I saw one DIY system that circulated water with a small bore tube inside the PVC pipe where the nipples were, with a pond pump and the afore mentioned submersible aquarium heater in the main storage bucket. I would think that would be the way to go in extreme cold, I think the pipe wraps would not be as effective, but that is just a guess on my part.
I have heard of people adding a bit of glycerine to rhe the water which lowers its freezing point. You mighy want to try it out after some research. Also, insulating the whole system is a must. If you want the heater to work it must be placed at the lowest point in the network else points lower than the heater will freeze