[rohrmann]

The cold wasn't that bad, but did you use the furnace at all over night? If not, maybe it got cold enough to freeze the drain line. If you only spent a couple nights in the rig, you didn't really have enough waste in the black tank to drain it. You should really have at least a 1/2 tank or more to get a good drain. I would wait until things warmed up more, run a lot more water into the black tank, and then try it again, along with running the black tank flush while draining it.