You can get a bike route map of Tucson which shows bike paths and routes through most of the city. Take Oracle Road (Highway 77) north out of Tucson to Highway 79 to Florence, then Highway 287 from Florence to Casa Grande. Then take Highway 238 to Gila Bend. There are frontage roads along much of I-8 west of Gila Bend; where they aren't present, ride the interstate shoulder. At I-8 Exit 54, take the paved road to Tacna, Wellton, etc. to Yuma -- the road avoids a mountain range that I-8 crosses east of Yuma.
This routing from El Paso to Yuma is high desert and desert that get very hot in the summer, with long stretches between water sources and other civilization. I've ridden virtually all of it and it's wonderful riding, but be prepared.
I haven't ridden from Yuma to San Diego, but you could likely find journals on crazyguyonabike.com of that portion. Not too far west of Yuma, you can pick up the Southen Tour route.