For the windows cover them with foil backed
Radiant Heat Barrier insulation. Basically bubble wrap with a foil backing. Can be roller up of laid flat when not in use. I store mine under the mattress.
When the temps really drop the furnace is going to run a lot (more on than off) and the blowers are going to demand a lot of kilowatts from your system. Stay on top of the battery bank and keep the standby generator ready for use. As long as you can keep the propane flowing and the furnace running your water system should be safe.
