From personal experience that seems extremely conservative, although that may be the point. According to [1], a semi truck or city bus use a little less than a gallon per hour idling.
[1] https://www.energy.gov/eere/vehicles/fact-861-february-23-20...
But, when planning like this it pays to be ultra conservative. Shit happens.
Basically it has to do with a diesel throttling the fuel and never having any restriction on the air vs a gas engine throttling the air.
I have a non-plug in hybrid, that gets almost 50 mpg consistently; I've got to experiment and extrapolate to see if it's much better. I would like to believe so.
I recently learned of the existence of:
https://en.wikipedia.org/wiki/Gasoline_heater
and wonder how efficient they are.
These seem to be mostly outdated, but I guess that they are particularly useful with air cooled engines, where a conventional heating system doesn't have coolant to work with.
Mine does shut off, if the vehicle isn't moving and the interior temperature matches the climate control setting.
So I would think/hope it could use a fraction of the fuel by running intermittently.
We always refuel before entering a risky part of travel. Never count on getting there. Make sure. And in a scenario like this that refuel could provide a days worth of heat, depending. No matter what, it's a lot more heat than driving at reduced capacity would have yielded.
Rough metric is you get 1 hour of engine idle per gallon. Some cars do better than that. And you can pulse it, warm vehicle up, make sure everything gets charged, fluids able to flow, etc... and then shut down for 1/2 hour or so in order to stretch the resource.
We carry:
-2 gallons of water
-Car road trouble kit, flairs, compressor, various tools, tape, things...
-First Aid
-Some food
And this is usually our road munchies, pet food, etc...
-Weather related stuff