I have several suggestions to consider...
Turn the trailer so that the ingress/egress side faces south east.
Add tarps in layers witH an air gap in between to block the wind from stripping the heat from the north/north west.
Skirt the vehicle and heat trace any external water lines.
Add a tarp to the top to add an insulating air pocket.
Use supplemental electric space heaters with thermostatic controls at each end of the trailer.
Use 3M heat shrink window film on the inside of all windows.
Open blinds on the sunny side to gain heat during the day and close them tight at night to retain the heat.
Run a dehumidifier as needed to keep the inside moisture below 60%.
Add a wind block air lock to the ingress/egress point if possible.
It’s all about doing small incremental things that add up to make it more comfortable.
