The locals to this area know that September and October are incredibly beautiful - and there is rarely fog this time of year. The campgrounds are never full in the late fall either. Camping in the California deserts, in Arizona, New Mexico, most parts of Utah, etc. is miserable and dangerous in the summer months. The wonderful desert vegetation blooms in early to mid spring. Summer is not a time to be in the desert.
Many states are incredibly hot and humid in the summer months.
My son was glued to me with sweat. Georgia, Alabama, and Florida are quite humid in summer too. I camped in Georgia in July and was forced to launder my clothing halfway through the trip because everything had mildew. Florida has terrible thunderstorms on the coastline facing the Gulf of Mexico. These storms are short but quite intense.