2 1800 watt hair dryers, no way first they cant be over 1500 watts and use a normal plug, but that does not mean he did not change a plug. 2, that is one noisy way to get heat. 3, thats 3600 watts at 120 volts, or 30 amps at 120 volts, then lets say he had an inverter running from the alt and battery and it was 100 % effecent, that would be 300 amps at 12 volts, or enought to drain the battery flat in 15 minutes even with the alt charging it.
what i used was a coleman heater designed for use in cars, as a emergency heater if you broke down in winter, i mounted it under the dash. between the seats aiming at the back, it was enough to help my leaky HE at 20 to 30 outside temps,
here is the one i used in my car. and i never had a problem the 914 is leaky enough to avoid CO stuff
but coleman also makes a 1100 btu heater, as they put it "for rec use in enclosed spaces. that would work by it self to heat a car