Don't cut down the 12 volt electric blanket until you've tried it. We have one and it comes with a built in timer for 20 minutes or 40 minutes no way to leave it on and drain your battery. It is great, you can plug it in to preheat your regular or flannel sheets for 20 minutes or if truly cold set it for 40 minutes and drift off into dream land and it goes off automatically. We also have a Memory Foam Mattress topper from Camping World that we placed under our Sleep Number bed and it makes a great insulator under the air bed. We camp in cold country a lot and we are nice and comfy. I have wired in a switch for running the 12 volt Electric Blanket.
Happy Campers
Gary & Laurii