Gary,

The I-10 route that John mentioned would keep you in the lower elevations.

But, if you've got an extra day, come down I-10 into Indio, and stay at the Outdoor Resort (or other nice park) for a night in the Palm Springs/Indo area.

Then work your way up to the 15, and head out to Vegas that way. Yeah, it's an extra day's drive, for sure. But, it would be a nice stay in Indio.

Ray