I can easily tell via my throttle pedal and 5.2 liter v8 engine response, when my depleted battery is gobbling up everything the alternator can make, versus when the battery is full and my solar wattage is partially feeding the Ignition and fuel pump's 12 amp draw.
But record fuel consumption differences, Ain't gonna happen, but each 25amps the alternator makes sucks up one engine HP, so there will be an effect on fuel consumed.
Measuring/quantifying it is another subject. Easier to dimmiss it and drive a bit slower of one is concerned about mpg's