If you can comfortably drive Phoenix to Flagstaff going to San Diego (Chula Vista), will hold no terror. There are a number of long 6%+ grades but Interstate 8 is in good shape. Take 85 south off of I 10 west of Phoenix and pick up I 8 at Gila Bend.
The worst part of the drive is that it is mostly boring. We live in Tucson, originally from San Diego and over the last 10 years have made that drive nearly 50 times. Fuel up in Yuma as California gas prices are far higher. I 8 from Dome Valley to the border, is a serious speed trap but doubt that would be an issue for you in your coach.
:C