As long as you can maintain a source of propane and keep the furnace running winterizing isn't needed. The onboard tank may only last a few days when it gets really cold so start now looking for a propane supplier that will provide a bigger external tank and delivery service. Electric space heaters help but you still need the propane furnace to pump heater air to your tanks to keep them from freezing.
You'll probably need a heated fresh water hose to prevent it from freezing. Also maybe a small electric heater in you wet compartment to prevent the black/gray dump valves from freezing. When you do dump the tanks ensure the hose is completely empty. Don't dump the tanks in the evening. The bigger the volume of water in the tank the colder it will need to be to freeze it. Best to keep the fresh water tank empty.
Head out to the local big box construction store and get a roll of radiant foil insulation. Looks like bubble wrap with foil on both sides. Use to cover the interior windows, windshield included. It makes a big difference.
Retracting the slides will help seal the coach up better then when extended. Helps reduce drafts and reduces the total air volume inside the coach you will be heating.
If the weather guessers are forecasting weeks at or close to freezing consider using hay bails or something around the base of the coach to help keep the wind and cold air out from below the floor. Will help to keep the tanks from freezing and the floor a bit warmer.
Good luck.
