When do you wrap the drain lines?

A question. Are your black and grey tanks exposed underneath the trailer or are they up inside/hidden? If they are hidden they are most likely in a heated space (provided you are using your furnace for heat and not just electric heaters). If that is the case you can drain the tanks, disconnect the slinky, and then insulate just the valve portion of the tank that is outside. The most likely part that would freeze would be the valve/slide gate right at the end. And that would take a lot of cold with the heated liquid right against it. Then when the tank gets full go out, remove insulation, and hook up the slinky and drain. Just make sure you drain the slinky after you unhook so nothing freezes in it. If your tanks are exposed ..... hope you have tank heaters installed.