Built in Tablet Chargers in the Van--Voltage regulation heat dissipation a problem?
Your math is correct. Although 940 C is a bit high, they usually limit around 120 C. You will still create a nice heater using linear regulators. Which is why people don't use them for a big drop in voltage.

Today we use switching power supplies.

And there are little modules as easy as those linear regulators.


They are more expensive directly compared to a single linear regulator. But it will earn itself back if you look at the whole project, and wasted energy during usage.