Keeping low point drain from freezing

We will be camping next week in Pigeon Forge and they are predicting a few nights of temps down around 17. We have heated tanks but our low point drains are exposed and in the past they have froze. In cases where it is going to be around 28-32 we turn on the heated tanks and let the water run a little in the tub to keep from freezing. But with the temps down in the teens I am afraid to let the water run in the event it freezes in the pipe to the sewer. So we are expecting to disconnect from the campground water overnight. Like I said in the past when we have done this our low points froze and I was out with hair dryer to unfreeze.

So long story short what has everyone done to keep these from freezing?

Thank you for any and all suggestions.