Here is something I found posted on another RV forum from 2012. Obviously this debate and the fuzzy answers from government officials in Arizona has been going on for a while:
"I called the State of AZ fuel information number and asked if motorhomes were exempt from paying the truck rate despite their weight or number of axles.
I got an honest answer. This is a grey area! A motorhome with two axles that is registered is Arizona is clearly exempt BECAUSE their Arizona registration says they have ZERO weight. I told her my SD registration shows over 34,000lbs. She said, as I said before this is a grey area, but in reality nobody is going to ask. Next I asked about 3 axles. Same answer - this is a grey area, but for now they are not enforcing the truck rate against motorhomes.
She confirmed that the stations that gave me a credit for the 8cents were correct and that I cold ask for the credit at any station. The law says they have to give you the credit if they don't have any pumps with the auto tax rate, but doesn't seem to make it mandatory if they have both auto and truck pumps.
So as of yesterday it was OK to fuel large rigs at the auto pumps, but who knows when a tax starved state will change their minds."