Depending on the amount of freezing, I am usually most concerned about the non-pex parts, such as the pump, and fittings. The pex might expand enough to allow freezing, but I wouldn't count on it. Of concerh are the drain lines for the waste tanks, those pipes are big enough to contain a lot of water which if frozen would most likely crack the pipes and valves. The tanks thenselves probably have enough space that bursting would be improbable. Personally, I hope that I never have occasion to determine an accurate answer to your question!
Noel and Betty Johnson (and Harry)2005 GulfStream Ultra Supreme, 1 Old grouch, 1 wonderful wife, and two silly poodles.