For highway driving, when the gas engine is running all the time anyway, you are better off to use MAX A/C or RECIRC. This allows the cold interior air to recirculate rather than being mixed with hot outside air. This allows the compressor to run less, especially as the car cools and you can turn the temperature higher.

MAX A/C and the RECIRC just above it on the dial are the same with the exception that at lower speeds the gas engine can shut down as it would normally. Use this in lower speed driving if you don't mind it blowing warm air when running in EV mode. Keep the fan at a lower speed and you may find the cold air lasts longer.

Also, avoid using the defroster as this runs the A/C compressor as well.

Enjoy your trip.