Not possible to stop exposed plumbing from freezing if temperatures are below freezing for more than a couple of hours unless the rig is built to handle it. Our external grey tank froze after a night down to - 7C (in Germany) and with daytime temperatures hovering around zero, there was no chance of it thawing. We had a heap of dehumidifier chemicals so I dissolved it in hot water and poured it down the drains. Worked OK and thereafter the tank was left open. Another trip we lost fresh water in the cold despite the mh being winter-rated. Turned out the pump suction line was run touching the wheel well moulding which was thin plastic. Pipe froze and I had to dismantle half the kitchen to get at it and reroute it. In the USA we have overnighted down to -20C without problems but only by running the propane furnace all night and then only because I had added a 4" hot air duct blasting down into the tank and wet bays under the floor. One thing to watch is your grey tank drain - if it has pvc pipe and a plastic valve on the end and has water inside, it is very likely to crack.