For Nevada, I think you'd want the second A/C and the optional dual pane windows which would really help keep the heat out. Hopefully some of our owners from the southwest can chime in.
I'm in the northeast and I had my 303 built at the factory with the Dual pane windows. Also my dealer installed a air conditioner in the bedroom. It gets hot {90 plus degrees} and very humid here. Not as hot as you guys but lots of Humidity! We have camped at the beaches here with 100% sun and I have no complaints with how my 303 Has Handled the situation. I was only able to use my main air conditioner because campground only has 30 amp hookups. When I camp at other campgrounds with 50 amp my 303 is really cold! Very glad I did both of these upgrades. For Nevada I really think you need both.