I just did a 3 months trip out west from August to October, 2 campers. NO reservation at all. It was a bit harder on the Oregon Coast and in California if you stay at ocean side campground, you could allways call a few days ahead. There is allways the free National Forest, wallmart, truck stop like FlyingJ, except in south California. Have faith, go with the wind, this is in my opinion the best way to travel, you like it you stop, you don't, you move out, never restricted by a schedule.
I agree that Death Valley could be on the very hot side, but sure no problem for camping! Some nice Casino Camping in Vegas on Boulder Highway with free shuttle to the Strip or normal town Busses.
