That looks like some serious burning of the rubber. From what I see in the pictures it seems to be confined to the top of the tire which would be most exposed to heating from the sun, even under a tire cover.
I see the high in Surprise, AZ in July could have reached 122 degrees F air temperature. Up here in the Northeast the highest temp I have measured on my tires was 110 F after 2 hours on the road on a HOT (for us) sunny day.
(P.S. - that second picture is so wide I have to do some serious side scrolling to read the posts. )