My 4L V6 Dakota burns ~.35gal/hr @ idle
That’s at idle, but idle won’t keep 900 watts going.
Yeah, my 80’s Ford diesel burns not much more than that, at no-load idle.
However, to power the 2500 watt inverter when the rooftop A/C is running off it requires a healthy dose of throttle or the banks of 7 starting and auxiliary batteries will run down. The alternator needs sufficient rpm to provide the charging current.