Lots of folks add some insulation to the inside of their popups in cold weather. You can buy this stuff in rolls ... it looks like bubble wrap between two sheets of aluminum foil. Cut it to size and zip it into the windows to keep the cold out.

An electric matress warmer or electric blanket might help too. I have a 12 volt matress warmer. It just runs on the battery, so I don't even need to be hooked up to use it.