Before we had the big class A motorhome with central heat we used a 12 Volt Electric blanket. It just plugged into a cigarette lighter socket.
There is also another 12 v type on the market that is a mattress cover and heats from the bottom.
check out the RV mags and stores or Google for these.
Everything I have ever read states that you should bring your cold equipment into a warm room and let it set and warm up for 2 to 4 hours before using?
Best to all
Bebop