I have stayed in my 2018 Classic 33 at temperatures as low as 5 below zero without skirting. My experience is you are good to about 15 degree minimum temps without doing much more than setting the inside temperature higher, running tank heaters, and disconnecting fresh water and sewer hoses.
I have invested in a 1500
watt portable electric heater as an emergency backup, foam insulating plugs for the Fantastic Fan vents, and I carry a spare Alde boiler circuit board, Alde control panel and Alde emergency start plug.
Make sure the outdoor shower is drained, and I recommend packing the shower compartment with foam or fiberglass insulation.
On really cold nights the first thing to freeze will be the water lines that cross under the floor inside the heated tank compartment. I set up my portable electric heater to blow warm air through the access panel under the file drawer; just to the left of the kitchen sink. The water lines to/from the boiler cross under the floor at this access location.
I will run the Alde on propane and both electric heaters on the cold nights. The two Alde electric heaters wonít keep up, so make sure your propane tanks are full.