Either South Mtn or White Tanks Mtn. They have a site on both.
571-585 would be Maricopa FD, which is dispatched by Phoenix. They must have been working mutual-aid with another department, if Phoenix was using fire channel 4 to talk to them.
BR575 is, I believe, the Goodyear/Maricopa Fire station in Mobile, unless they've changed things around in the last 3 years. Depending on where their call was, it could have been South Mtn or White Tank. South Mtn is the default for Maricopa, but if they were out near Gila Bend, they may have needed to use White Tank. It's my understanding that Mutual Aid channels are Narrowband, making it even more amazing that you heard them in Yuma.
I don't know what the TPO or ERP may be. You could probably look it up on the FCC ULS page. KOA-519 is the main callsign for the VHF channels.
I wonder if there was some ducting or other propagation enhancement at work in your ability to hear it?
If I recall correctly, White Tank transmitter is 50w and South Mtn only 30w or 35w. I haven't worked in the Alarm Room for a couple years, so I don't know if they might have put up more transmitters on the mutual aid channels since they went narrowband. I'd say the signal found a hole in the mountains between Phoenix and Yuma, and you were in perfect alignment.