My electric heater is much higher power because I have almost no insulation, but going by the rated capacity of 375 Watts, it should run about 3 hours for each fully-charged 100 Ah lithium battery assuming nothing else is on. A van with 600 Amp-hours of 12.8 Volt lithium battery (roughly 7,500 Watt-hours if rated as useable capacity) should run that heater up to 20 hours.The low setting is just 375w! How long can you run it without draining too much of the Battery?
My estimate for future is twice that much power, or 750 Watt heater, which could run overnight (10 hours) on 600 Ah of battery. We don’t camp in very cold weather, so 750 Watts should work if combined with lots of insulation.