I second using a small electric heater. Any will do. The 'pod has 2 inches of insulation all around so it doesn't need to run much. We found when we camped last fall the bigger issue was condensation from breathing. I recommend leaving the bathroom vent open an inch and a window cracked a quarter-inch to get some circulation. I'm thinking about cutting a 2 inch hole underneath the shower and installing a small biscuit fan so I can keep the window closed and get a small draft going (the bathroom vent fan is overkill for this even on 1).
Winterizing and leaving it is an option many take, so thanks for bringing that up tdrace.
Somewhere here I posted a step-by-step on winterization. Don't have time to search for it, I have a meeting in 5 mintues.