Where in Arizona are you? We're up North, near Flagstaff, and I'm happy to chat about options we've found. Feel free to send me a message and I'll give you my cell number.
The short summary is, like RivetBoy said, registering it as an RV/MH is your best bet. We've used Progressive (for a "commercial vehicle for personal use") policy while the bus was still a bus, but that's more expensive than RV or motorhome insurance. We also used The Hartford for that in the past, but then they quit issuing those.
For a converted bus, we've used Progressive and National General and Farm Bureau at one time or the other. I understand that Allstate will also issue for those, but we're pretty happy with Farm Bureau for now so I haven't looked into Allstate myself.
Your conversion doesn't have to be fancy, you just need some of the items on the Arizona MVD list. For our first one, I used double-stick tape to adhere a Coleman stove to a counter for the "cooking" option. Then when we got it home, we ripped it off and moved on with the real conversion. Now, we've gotten all fancy and have a motorhome conversion module that we carry inside the bus, screw down, and drive to the inspection place. We have had really good luck with the third-party MVD offices in AZ.
If you're close enough to Flagstaff, we could probably help you with this...but it's easy to do yourself. Once you have a MH title, you're golden.