Well, if you start from Los Angeles and head northeast into the Mojave about the start of May, you may encounter only warm weather in the desert instead of blast furnace hot. That time of year, it's a very dry heat, also. The miserable monsoon humidity with hot nights doesn't kick in until late June or early July, usually. Often there's strong tailwinds in May, too. Most of the route from LA to Las Vegas is at least several thousand feet elevation, which does make some difference in temps. A side trip to Death Valley is then an option, depending on the weather forecasts.
This then means skipping Yosemite and Sequoia...hmmm. Perhaps you could ride from SF to Yosemite Valley, then take the Amtrack bus connection to Merced and take the train to LA to resume the trip from there. You can stay one night at the North Pines backpackers campground in the valley, then you have to find another site if you want to stay longer. The bus and train take bikes, no problem, just make sure to have your tickets first. (Buy online, pickup at any station with ticket service) It's still coast-to-coast, starting in LA, and Yosemite does have several groves of Sequoia trees, just so you can see them, too. Riding through the Central Valley should also get you adapted to warm weather before the desert, too. Other tricks for desert riding include rising very early and getting most or all the day's ride in before the sun reaches its peak, then lounging around someplace with AC or a motel/campground pool for the rest of the day.
Last edited by stevepusser; 10-12-12 at 10:26 PM.