Thats a bit colder than I expected. I still ask, how much ventilation does that trailer have? That level of frosting indicates higher humidity.
At those temps you cannot completely eliminate frost formation in the front cab. As long as there are breathing humans at least. You could try to fully isolate the front cab, with zero airflow, that would reduce it a fair bit. But every time you drive the vehicle you will warm and humidify the cab, which may create frost when it cools down.
If you expect to encounter temps colder than -10F on a regular and extended basis, using the front cab when parked is a non starter. Most vehicles simply do not have enough insulation. Even with the engine powered heater running at full you will have major cold spots near the floor.