I have exchanged email with Brent from Heatline
I asked for something to protect the black water discharge line down to about -6C (+20F) using battery power with solar.
He recommended a 12v 5' Kompensator
cable with lots of insulation. It comes pre-assembled and produces more wattage as temperature drops. At 20F it appears to produce <>3.5 watts/foot . They want $205 CDN for it.