Avoiding freeze-ups while towing in very cold weather?

Is the underbelly reasonably insulated and fully sealed against wind infiltration while under way.

Otherwise blowing a little bit of warm air down through a 2" duct isn't going to keep up with heat loss. Dumping both tanks every morning before heading off and adding some RV antifreeze will protect your dump valves, but other items such as external showers and tank drains and tank fill circuits might still be vulnerable.