Lots of insulation, dual pane glass, inside covers for the ceiling vents/skylights, heated tanks, and an enclosed area for the tank drain valves and water hook-ups. This last one is hard to fund on trailers, but it makes it much easier to prevent freeze-ups at the tank valves. There is always water on the tank side of the dump valve, so exposing it to outside air is a risk.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL