We have a duraflame powerheat infrared quartz heater from Sam's years ago. We use it in the 5'er from March or until it gets warm. Then, in the fall, we use it on cool evenings/mornings until we winterize after Christmas.
It may get below freezing and snow in Oct, Nov or Dec, but unless we have a constant below freezing for more than 24 hrs, we have never had a problem. As long as it warms above freezing during the day and the interior warms above 50, we don't winterize. This heater is set on it's lowest setting and we open the cabinets that house our water lines (kitchen sink, bathroom sink and door to the toilet. We had a lightbulb that we burned in the storage, but it burned out and we just have not replaced it. We may do that this fall, but we will see how cold it gets.
We usually do a week-long trip around Christmas and before we return, wherever that may be, we will winterize.
Our 5'er is winterized for 3-4 months, according to when Easter falls and we have spring break.