I don't have any personal experience with camping in really cold weather. DW is from Vernon, but we're on the coast where it rains non-stop all winter. I'd take cold weather over rain any time!
Are you using a heat blankets for the underside of tanks like this brand?:
I do know though, that sewage effluent doesn't freeze. That's what civil engineers have told me, so I wouldn't worry much about that one. I'm guessing that's not the same for grey water since it's only got soap in it. As long as the heat tape around the outlet pipe from the tank has some insulation on it, I don't think it matters much if it's in a straight line or wound around it. Even if you are skirted, is that going to keep the tanks warm enough? Is your underbelly enclosed and heated? That only protects down to around 6-7 deg. C. Some insulation on the vertical sides will probably help a lot. You could possibly fashion some of the insulation to be on the exterior side of the valves with a door of sorts. And then put a couple of incandescent lamps in there to make a little heat.
You could use a little RV antifreeze in the tanks but that might get expensive after a long winter. You wouldn't need as much though. I'd be using regular white 3" PVC pipe from the tank outlet to the inlet in the ground. It'll stand up better in cold weather, flow better and not get liquid trapped in the ridges.
What are you doing with the hose between the valve and the sewer connection on the ground? Is that heat taped and insulated? I'm wondering if you could just leave the grey water tank(s) valve open all the time and let the grey water drain freely into the inlet on the ground? That way the water flowing into the sewer is near room temp. This way you *could* also let the cold water supply drip a tiny bit continuously without worrying about the holding tank filling up.
Out of curiousity, what are you using for heat? The propane furnace or an electric heater or both? How well insulated is your trailer overall? I installed a permanent 1000W fan-forced heater in our old 20' TT and kept the heat on the lowest setting for a couple of months over the winter while doing a bunch of interior mods. It ran continuous due to poor insulating. Then we got a Hydro bill (2 months) nearly $1,000!! Won't be doing that again.