With a truck that has 1MWh of batteries (ten S-100D batteries... well over 5 tons of batteries) that is required to make the Class 8 truck go 333 miles, I suspect cabin heating is an afterthought. Heck, they could use a simple heater that burns diesel, that every truck stop will have. It can supplement battery heating, too. It doesn't need to be very big; just enough output to be a base level of heating for extreme cold. Of course, if things catch on, then the truck stops will have methanol, or hydrogen, or natural gas or propane or any number of other sources to power a small heater.
By the way, the diesel motor and transmission is over 1 ton, I think. The heavy drive axles are probably another ton. The rest of the truck is pretty light... aluminum / fiberglass / plastic bodies, and two simple C channel frame rails. I'll bet Tesla has to change the frame rails to save any significant weight. I suspect a unitized body / frame with air suspension to hold the "fifth wheel". No matter what, without relatively light weight capacitors being used, the truck will be TONS overweight.
The aerodynamics is where Tesla already shines, so I have little doubt that any truck will be one of the lowest drag units out there. They need to get rid of those rear view mirrors!!!
The biggest mistake Tesla can make is to build the truck too "dainty". I have welded frame rails on "brand name" trucks that cracked under load. I suspect this is an area that saving weight will get a really bad reputation. Everybody expects the range to be less than a diesel, so that won't be a surprise, and they also expect it to improve over time (more range, quick refuel), so this won't be the shortcoming.
The Tesla truck needs to be as good or better than a Freightliner, Mack, Peterbuilt or Kenworth at a significant savings in fuel costs. Tesla will have to give it away to compete with the hydrogen truck guys (Nikola).