They are dispatched by Mesa FD's Regional Dispatch Center, and operate on the Mesa FD talkgroups.
Yes, you will have to listen to all dispatches. You'll just have to train your ears to listen for Rio Verde's 441 and 442 apparatus numbers.
Be aware, that only the dispatcher will be heard on the TOPAZ RWC Thompson Peak (and Shaw Butte) IR sites. In order to hear the mobile/portable radios, you will have to listen to the TOPAZ RWC Mesa simulcast.
I have to assume they will also be using Mesa FD's analog conventional VHF channels when they come fully available. Dispatch is currently on 156.2400, with a DCS of 411.
John
Peoria