I've heard of a solar powered warming device. As long as there's been enough sunshine during the day it will warm up. Then place it on top of the bed cover and place a thick blanket or duvet, which is called a doona in some places, over it to keep the warmth in. It will however work much better if slipped under the covers. This device is very cheap and can even be used when camping.
What is it and where do you buy it? Some hardware stores have it. It's black all over, fairly thick and needs a bit of care during use.
Guaranteed no electricity required.
Okay, it's one or more large, strong and thick black plastic bags. Some hardware store stock them as garden refuse bags. Put some water in them and seal them. Lay them as flat as possible out in the sunlight. During the day they heat very nicely. Before dark take them inside, place them on the bed and cover them to trap the heat. Works a treat. Just make sure you don't prong them with anything sharp. Never let a cat in the room or you'll end up with a wet bet.
I even heat water for a shower by running some black agricultural pipe around the yard. Filled with water it heats up great. You can transfer the water to one of those camping showers or attach it to a pump and run it through a window etc. Free hot water. The kids even suggested putting thin ag pipes on the roof to get even more free hot water. But it wouldn't look very good.
Anyway, if it's too much trouble for most people to heat a bed this way, it's really no hassle when camping.
Remember, make sure the water doesn't leak out.
Hope this helps.