First off - insulation works the same in cold weather and hot weather - just like your house. More is better.
I stayed one winter in Phoenix. It was touching 100 degrees during the day as I was heading out on 1 May.
One observation I have is that I saw many folks with an open air car-port type of covering over their trailers and mobile homes. Just 4 poles and a hard covering. The idea is to just keep the sun off the rig and from directly heating up that big tin can. That is the first thing I would do.
I talked to a couple folks who stayed year round in the RV park I stayed at. They put insulation in the windows and bought a couple of household window AC units. They told me the household ones are much more efficient then the dedicated RV units.
