You said you did the spray port. Just to confirm - I have 3 things that could be considered spray ports. There is the outside shower, there is the hose and spray head in the inside tub/shower, and there is the kitchen sink sprayer. You need to ensure you clear all of them. Also, I recommend a quick hit of air throught the black tank sprayer (if you have one). Mine has a dip in the hose and it is a good place for a little water to collect. If it collects there it will freeze and split there too! I wouldnt worry too much about the black and/or grey tanks at 30, but if it is going down any lower I would ensure the tanks are drained too. It is far easier to drain the tanks than it is to repair or replace one that split.