To answer the OP's question....it's all about TAXES....in Arizona diesel fuel is taxed at two tiers. One for light vehicles, 18 cents/gal and one for heavies, 28 cents/gal. Here's the breakdown and info from the State of Arizona...
Fuel-Tax Rates in Arizona
The Arizona tax on gasoline (motor vehicle fuel) is 18¢ per gallon. Use fuel (diesel) is taxed at two rates in Arizona.
If the use fuel is used in the propulsion of a use class motor vehicle on a highway in this state, the tax rate is 26¢ for each gallon.
A "use class motor vehicle" means a motor vehicle that uses use fuel on a highway in this state and that is a road tractor, truck tractor, truck or passenger carrying vehicle having a declared gross vehicle weight of more than 26,000 pounds or having more than two axles.
If the use fuel is used in the propulsion of a light class motor vehicle (such as a pickup truck) or an exempt vehicle (certain school buses, government and non-profit vehicles, see 28-5432.C) on a highway in this state, the use fuel is taxed at the same rate per gallon as the motor vehicle fuel tax (18¢ per gallon). (See 28-5601, 5605, 5606.)
If you plan to fill up using the truck lanes, make sure you tell them you're an RV or you'll end up paying the extra tax.....Dennis