Also, if you can have a pet, the warmth coming off their body can give you warmth and vice versa.

I found Oodies to be excellent when I lived in a cold house with bad heating , even slept in it some nights

A bit off-topic but you can help keep the inside of the house a bit warmer by sticking bubble wrap on windows.

If you do plan to sleep in there, warning about mattress size - a standard single/king single mattress is just a bit too large to fit in a 2 person tent.

A single or child sized mattress maaaybe if it could fit through the entrance. It would be a really tight squeeze. But I think mine is a king single and could be a touch too big, and I would risk tearing the tent getting it through the opening. So either get an airbed/mat that’s small enough or buy the tent size accordingly.

If you plan to sleep just one person and don’t have a lot of room you can buy single person tents which are often basically swags. There’s not much room and they could be claustrophobic but they will heat up fast with your body warmth and there should be enough room for phone in bed.

https://www.kmart.com.au/product/pink-swag-43619511/

https://www.bigw.com.au/search?text=Single+person+tent&sort=price-asc

Ps. The floor is cold. Air mattresses are also kinda cold. Put them on the floor to let the air absorb the cold shock from that, then put like a yoga mat or folded blanket between you and the cold air. Even some cardboard. This is more comfortable than putting the mat under the air mattress.

If you have a sleeping bag, use it.

The more insulation between you and the ground (and between you and the air mattress) the better.

Small electric heating pads, like for cramps or back pain work really well to warm up small areas like sleeping bags or tents. They are pretty light on energy use.