I haven't done much winter tent camping, but what I did on the few occasions is gather lots of leaves and pine needles and make a pile, then put down a space blanket as a footprint and setup my 2-person backpacking tent over all this. The tent is 3-seasons so I setup the fly without the ridge pole to lay flat on top of the screen to cut down on airflow. Then I drape a sil-nylon poncho over the top and tie it down for a little better air tightness.

On one occasion my son and I spent two nights like that in temps that went down to the mid teens. Our sleeping bags were synthetic lightweight mummy bags rated for 20*, and we had regular thermarest pads. Filled our water bottles with boiling water and used them to pre-heat the foot of the sleeping bags, then wrapped them in our extra set of underwear and used them like a hot water bottle.

We had small, single-mantle gas lamp and backpacker's stove, but didn't use them in the tent at night for fear of setting ourselves on fire or getting CO2 poisoning. We did burn two candle lanterns, but I don't know if they really did much for heating.

It wasn't toasty inside the tent, but it felt at least 20*F higher than out in the wind. We slept in our clothes, thermal underwear, double layer of wool socks, gloves and hats. We also wore our rainpants and extra shirt, and draped our jackets over our legs and feet and used our second poncho on top to try and catch some of our body heat. We were actually pretty comfortable in our bags.

The worst part was getting-up in the middle of the night to pee. And waiting until we got the fire stoked the next morning.

We didn't have chemical heat packs then, I don't think they even existed. Today I'd bring a bunch and place them in my sleeping bag for warmth. Or better yet, just make sure the batteries are fully charged in the camper, turn on the furnace and plug-in the heated mattresses. Holiday Inn on wheels!