December 2015 I spent a month driving from Michigan to Utah, hit nine National Parks there and then onto Death Valley.
Coldest temps I experienced were in the plain states... Lowest was -15F.
I didn't winterize, instead doing battle with old man winter.
All exposed sewer plumbing is wrapped with aluminum, then heat tape, more aluminium and finally reflectix. Fresh line from the pump also heat taped. 240 watts of heat pads on exposed holding tanks. Finally hot water recirculation system keeps the rest of the water system limber.
In the truck bed a little blue generator worked tirelessly, only taking breaks for fuel or when temps were above 40.
First few years I did true dry camping in the trailer, not even using the waste tanks... No more of that!