I think one would have to look up the legal definition of Fifth Wheel in AZ, including whether or not case law had found one way or the other on gooseneck hitches (which is essentially what the
hitch is, except it uses a smaller ball than a standard gooseneck). I believe the reason the law states 5W is to preclude two bumper hitches in tandem, but that's my opinion.
Roger's advice is right on! Have the papers to prove your point because this is certainly going to be a gray area in some places and if you are involved in an accident that is not your obvious fault, it may save you from being cited and appearing to be at fault.
Some states allow it, some states allow it only on federal highways and within x miles of exits and some states forbid it. Apparently it doesn't fall
under the Full Faith and Credit part of Interstate Law.