I don't have any yet (thinking about buying some though), but folks have good things to say about PopupGizmos. They are best known for keeping the heat out of the camper in the summer by reflecting the sun rather than letting it heat up the bunk ends. Their web site says that they help keep things warm in the winter though. PopupGizmos
I'm in Texas and more interested in the "keeping heat out" aspect of them.
Our Roo came with heated mattresses.... Maybe pack an electric blanket to help if you have electric hookup?