Once you get the sewer thawed out drill a hole in the clean out plug and thread a heat tape down the sewer pipe till it goes under ground. It will keep that pipe from freezing. You then seal the hole up with caulk.

again, but this is another example of how inappropriate trailer-steading is for your new climate. I hope you'll consider, both for your comfort and for the environment, building an envelope of insulation entirely around the trailer, and especially above the roof. If you did it yourselves you could do strawbale or slipstraw with minimal framing---just enough framing to build another roof over the old one (which could include good overhang for new walls).