All come with a 100 kWh battery. If you have the Long Range variant – which has an impressive range of 412 miles – it costs $15.29 for a full charge based on electricity prices of $0.13 per kWh and 85% charging efficiency.
https://bikehike.org/how-many-watts-in-a-tesla-battery/
Naturally that is assuming you are running a 750 kWatt heater Continuously and that the cab has zero insulation- which are pretty obviously false assumptions
You are underestimating how good modern lithium carbonate/perchlorate batteries are, how long they last, and how efficient they work - which is understandable. Most people have experience based on metal hydride, or Nickel Cadmium batteries.
The batteries in a 2020 and beyond Tesla should maintain 80% Charge capacity after a million miles worth of charging
How about opening the door occasionally. With a battery powered system, the heat comes from the power/energy used to move the vehicle down the road. That means that you don’t go as far if you use the heater.
With a gasoline based vehicle, the heat is extra, a freebie. It costs nothing extra. Well, actually you try to get rid of it.