Some great tips here. Here are mine that have not been mentioned.
* The first week I got our RV I had the driver and pass windows tinted as dark as legally possible. When you live in the Southwest tinting windows is not a fashion statement, it really is needed. I will never own a vehicle without tinting. The small glass above the drivers and pass windows is also tinted "limo".
* I also picked up some white insulation material at Walmart. The ladies use this stuff for making pillows, puppets, ect... I used this as fiberglass is scratchy and I did not need alot. I put this material inbetween my shower Skylight. As simple as moving the inner panel in the shower and shoving the stuff inside. Cuts down on the light a bit but it still comes thru.
* The other area I put the white stuff is above the front seats in that wasted entertainment space behind the TV. There is zero insullation in there and the top of the front windshield is black. Talk about a heat magnet.
* I also make sure AC radiator on roof and vehicle are clean before my summer trips.
Other tips mentioned that I do.
* Install pillow in roof vents.
* Pull shades down
* Start roof AC before it gets hot outside.
Remember every passenger inside your vehicle is a heater.
I always questioned the dash AC vs the roof unit mpg. Not sure how much the AC hurts your MPG when most of us are getting under 10mpg. Awhile back on Mythbusters they did a mpg test in a car. AC vs Windows down. Windows down hurt MPG more than AC. Also I believe my dash AC is alot colder than the roof unit.