For cold weather camping, I added a fairly thick emergency blanket on top of my RTT mattress, and under a wool blanket. I was surprised how much difference that made. This works just the same if your sleeping on the ground or in a teardrop (or anywhere, I guess), it just reflects your body heat back up. You could drape another over your sleeping bag too. Also consider layering, a toboggan, a decent quality sleeping bag rated for your comfort level and the environment, wool socks, and those hot hands packs like others said - all this goes a long way.
None of this involves gas or electricity and is generally budget friendly. I camped with my young kids (3, 5, 6) in mid-20s in our RTT with the above setup and I couldn’t believe how well they slept. Trial and error, figure out what works for you. I’m just not a fan of the potential fire hazard of electric blankets in camp, or in the house. Hope any of this helps!