We install a product by the name of Webasto Blueheat at work. I am not trying to sell these for them just so you know.
They actually are an amazing product, they act like a remote start by warming your engine and interior. But without running your engine, they have small unit that heats your coolant and runs your interior heat. They use fuel from your vehicle to do so but only .16gal/hr on full and .08gal/hr on reduced.
Here is some more info on these, we put them in the Ambulances we do remounts on.
Blueheat
They also have
interior heaters that could possibly be put to use with an EV! You would just have to have a very small fuel supply tank for them like maybe 2 gallon or less. They only burn .06gal/hr on full and .03gal/hr on reduced.
Just though I would share.