Just looked at my copy of the maintenance manual for a fleet of 40 footers I bought around 1984. Maintenance manual states that "standard tire size is 12.5-22.5".
But, manual also presents a load capacity chart for a variety of other tire sizes, ranging from 11.00-20 to 12.75-22.5.
So, most likely 12.5x22.5, but not guaranteed.
Arthur