Trees in Nevada? You have never been here Jim?
Ely is not going to be much colder than Las Vegas, while shortest route you plan is via Las Vegas.
Nevada just started opening the parks and casinos are due next week, so you have to keep the eye on news. We drove 93 a week ago and all parks there are still closed. So far all campgrounds are closed.
If Las Vegas is too hot for you - Mesquite used to have very nice RV park for good prices.
Still no matter how you go, this is not area where you can sleep without AC.
On positive side, we are getting colder weather in Vegas now. So far only 100F after 113F wave midweek.