Perhaps my experience is different, since I have an enclosed sleeper, I am not heating the entire van.
Also, I have not used it at extreme cold yet. Perhaps sub zero will have me crying a different tale!
When I go to bed at night, I do not want it "hot" anyway ... if the temperature is somewhere above 40 degrees in here I'm a happy camper!
But as a matter of discussion, if all you wish to run with the generator is a heater, why invest in an "inverter" generator in the first place? Those such as Honda and Yahama produce a "clean" ac current, to run sensitive electronic devices. A much cheaper generator will run a heater.
Also, my generator, when operational, powers a converter that produces 50 amps, (I think), of dc power to charge both my batteries, truck and inverter.
The inverter runs the electronics with clean ac.
I bought the inverter model of the generator for a different reason, it is more compact, quiter and more easily stored. Otherwise I would have bought one of the $300-400 models.