Coldest you have camped with no heated tanks.

We just got back from Utah where we had five nights of temperatures in the nine or ten degree range every night, with highs in the high 30s. No major problems. One night, the line to our cold water faucet froze, and it thawed in a few hours. We did have hookups. We don't have heated tanks, but the trailer is very well insulated. We did not use the heater at night -- it was in the low to mid 20s every morning when we woke up. (Lots of blankets!)

I think that if the daytime highs had been below freezing, we would have had serious trouble.