The 2021 and 2022 ATV are both rated as 1A, 15W.
What is your battery bank? Do you know the rated voltage and amp-hours or the watt-hours? If you know the watt-hours, then the math is simpler - 15W draw for 15 hours would be 225 Wh.
I have an RV with a 12V battery system and run an inverter. Using the math equation Watts = amps * volts, I figure the ATV actually runs on 15V (15 = 1*V or V=15/1 which is V=15). Knowing that, I can use the same equation to figure out how many amps it will draw from my 12V battery: 15 = 12 * A or A=15/12 which would be 1.25A. So for me, if I wanted to run it for 15 hours, it would draw 18.75A from my 12V battery.
I think the 1.25A on a 12V battery is ballpark, it may use a little less - I only have it on when I have the TV on so I haven’t tracked just the ATV by itself. But watching how many amps I use when I watch TV for 4 hours in the evening, it’s probably close to that.