Laws do get changed and modified.This is the latest revised statute I could find on this subject. I will do a bit more research.
Arizona Revised Statutes (ARS) 28-5708 imposes the use fuel tax on light class motor vehicles as defined under ARS 28-5701 and on exempt vehicles pursuant to ARS 28-5432.
“Light Class Motor Vehicles” are defined as any motor vehicles using use fuel (diesel) on the highways of this state except for use class vehicles. Use class vehicles include a road tractor, a truck tractor, a truck, having a declared gross vehicle weight of more than 26,000 pounds or having more than two axles or a passenger carrying motor vehicle designed to seat more than 20 occupants."
This is copied from the state law. I read it as two axle MH over 26K are classified as Light Class Motor Vehicles.