Here's an interesting thread on this very subject from 2012:
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/25936142.cfmPeople on this thread are saying RV's are exempt, yet nobody offers legal authority for why they say they are exempt. But what I found the most entertaining was the second to the last post in the thread posted by "lbligh" and states the following:
"NOT true. The AZ fuel tax being discussed applies only to commercial vehicles. RVs are exempt. If you fill at a truck facility you simply go inside and advise them that you are a non-commercial vehicle and they will deduct/refund the $0.08 per gallon tax."