Heating question.

I am new to Airstreaming & RVing in general. This post might get a little wordy but I want to give as much info as I can so you folks will be able to give me the best advice, so forgive me. I purchased a 2007 Safari 27FB SE a little less than 2 months ago. My question has to do with heating. I am lucky enough to have the trailer hooked up to 30amp service 24/7, even when it's not being used. For that reason & because my wife & I are taking short weekend trips when ever we can, I have not winterized it. Most of the time I keep the thermostat set on Heat Pump at 50*. I live in North Carolina. Not the coldest place in the world but it does get down in to the 20's at night fairly often. I have read that the heating system will automatically switch from heat pump to furnace at around 40*. So my first question is, is that 40* outside temp or inside. I have asked this question to a Airstream Corporate customer service person. After listening to him read,under his breath, from a manual, he told me that it was inside 40*. So that means having the system set as I do, the furnace will never kick on. This troubles me because I have read that it's the furnace that keeps things under the trailer from freezing. So if the temperature outside is getting to the 20's or sometimes even colder, am I going to have a problem? Now I say I haven't winterized which is true, but I have drained the tanks & lines as best as I could except for the black water tank which I have left about 5 gallons of water in. So to summerize, does the furnace switch at 40* outside temp or inside? If the temp outside gets below freezing & the furnace doesn't turn on will that be a problem? I have had people tell me to keep it on furnace full time. But that, as you know, uses propane which can get costly. The electricity that the trailer is using is free to me. So folks what do you suggest? Thanks