Although 28 t0 30 is sub-freezing, if its just for overnight hours thats not cold enough for long enough to freeze. Just my opinion, but based on many years in Chicago with many campers, we have left water in many lesser quality campers under the same conditions and have never froze anything. I am not by any means suggesting this on a regular basis and be carefull but if those are the lows at night, not to worry.