Lithium Battery heater

Major brands of Lifepo4 batteries are available with internally heated models (eg. Battle born, Lithionics). If you're doing an initial setup, this would be the way to go.

Use Lithium Titinate batteries, or SiO2

I went another route.
I added a small fan with a temp sensor and digital readout and the fan comes on when temp gets down to 40F and circulates the warmer air on the inside of the cabin into the battery compartment.
Added a couple of small vent covers to allow the air to flow easier.
We had 30f the other day here in SD if you can believe that and I ran outside and checked the temp in the battery compartment and it was 50f.
Temp inside the cabin was at 52f.

3 tons wrote:
Get yourself a wireless Thermometer with remote sensors from Amazon, place one sensor on the battery…

3 tons

Do that.
And insulate the exterior battery compartment door.
If possible, leave interior cabinet door(s) open so that cabin air can circulate into basement area.
You may not need a battery warmer unless you plan to camp in extreme low temperatures and charge battery in those conditions.

Get yourself a wireless Thermometer with remote sensors from Amazon, place one sensor on the battery…

3 tons

I was looking at THESE but I ended up mounting my battery inside the camper so the furnace keeps it warm. I may concider these when I do the 5th wheel. these are low power so you would definatly have to have the batteries in a insulated container for them to be effective.

you would also need somthing like THIS to controle them.

Go to an automotive parts house. Battery plate and battery wraps have been made for over 75 years. They are in the same department as engine block heaters. Check on Amazon, should be between 30.00 to 100.00.

Tank heaters.

Battle Born Batteries has heat pads for sale. They aren't cheap though