I attached an aero cool to the side vent door on our fifthwheel. I made a boot out of bubble foil to connect the cooler. Best mod, kept us comfy in the desert heat. 115 outside, 65 inside on low, with every window open and the door. It saved our lives, and made life so much more enjoyable. We could sleep at night, the humm from the fan I miss alot. We would open the fantastic fan above our bed and turn it on low, it would drag the cool air into the master bed room.
But...I was able to install ours on the side of our rig, on blocks. I would never install it on the roof, if I traveled with it. The high profile will cause it to loosen up and fall off your rig.
Buy a small swamp cooler, and rig up a way to put it in a window. That way you can handle it easier. A swamp cooler in a hot,dry climate is perfect. They use alot of water tho.