What are you trying to accomplish? If you can camp without water, you can camp in ANY temperature. The furnace does a pretty good job, an additional electric heater can supplement that. In cold weather we tend to use a heated mattress pad.
If you want to have full use of water, that's a different story. Without an artic package and heating pads for your tanks (fresh and waste), I wouldn't camp in too cold temps. If it gets to upper 20's at night and warms above freezing during the day you should be okay. If it stays below freezing all day, or gets into the teens at night I would start to worry.