One Frozen watee line

So I boiled hot water and dumped Rv anti freeze down the drain and it was unthawed with in the hour. We keep the heater on constantly that why I can't understand why it froze all of a sudden because its been a lot colder here than last night. Our water hose and sewer connection are all heat taped and we hard piped the sewer hose and wrapped it as well. It wasn't the pipe itself that was frozen because I kept the bathroom running all day but the Grey tank or its outlet was the culprit. Everything is in working order now. And my carpet is drying. Thanks for the help everyone. I was in a serious panic today!