Being a desert dweller myself I would suggest you stay out of Death Valley and Las Vegas in the summer. Staying overnight would be a real challenge as the nighttime temperatures could be well over 90 and an RV A/C unit will not be enough. Entering Death Valley from the west on CA190 anytime of the year with an RV without an engine brake is not recommended. Death Valley is a land of extreams, heat, steep terrain and very desolate. I have done it in July with my DP towing a trailer but then I live in a similar environment so it was "normal" to me and I probably wouldn't do it again.
Use clarkster's route or take the long way around by not going through Yosemite to US395, but loop through Yosemite valley then head south on CA99 to Bakersfield and east on CA58 to I15 into Vegas. But remember that Vegas is desert too and it will not be much different than Death Valley. These are areas best visited in the winter.