San Diego to Phoenix in August eh?
I did the run between LA and Phoenix in August once.
It was 109 degrees at 9 AM when I left Palm Springs and got up to 119 degrees by the time I got to Phoenix.
Little pink skinned Michigan boy burnt right up out there. I think it was the most hostile environment I ever rode through not so much for me but it killed the air cooled bike (HD) I was on. I had a 2.2 gal gas tank so I needed to ride it during the day as there was no gas stations open at night. I was running 3 qts of 70w oil and 2 cans of STP and the oil was still like water.
Oil had burnt on the valve stems and later when I got up into the Mountains of Colorado it went clank clank and that was it.
Did learn a few tricks down there though. Get your clothes soaking wet and put your rain suit on so the water won't evaporate so fast to stay cool.
Don't put an extra gal of gas in a Prestone bottle and bungee it to the sissy bar because it will get to about the size of a basketball. When I saw it I took it off and offered it to a family of migrant workers in a station wagon but they didn't want it because it was Premium and their car ran on Regular so I went behind the rest area and poured it out on the ground. 1 whole gallon and it never got the ground wet.
Take lots of sun screen, my lips and nose looked like the dried up mud that was everywhere. Dry and cracked.
Every uphill grade had a steaming car off on the side of it.
The locals just laugh at ya while your dancing on the griddle!
Yup, too fricken hot for me that time of the year.
My 2 cents, avoid the Mohave like the plague during the summer months!
Aside from that enjoy your trip, wish I was going with ya.