Plumbing system winterization - when should I be conerned?

You should be alright. If you are really concerned and you are hooked into shore power just put a trouble light in the area of most of your pipes and leave it on over night. You would be surprised how much heat a light bulb puts off in a semi-enclosed area. We always kept a light bulb burning over the winter right by the well pump at my parents house. When that baby went out the water stopped flowing.