Dozens of gallons of near-boiling water (it was boiling before we made the trek from the house to the cabin) and hours of plunging later and the drain still isn't working. Unfortunately, there are no access points to the plumbing inside the cabin. There might be a way to get under the cabin, but it's not a job that I am willing to undertake in this weather. Fortunately, we are supposed to warm up to near freezing in the next week. We will check on it in the meantime, but me might need to wait until then (or even later) before we can thaw the drain. I pray that none of the pipes broke due to this freeze.