Time to cut the big hole in the roof! I used the adapter spacer from Hein on the promaster forums. It sure makes it easy to do. It fills in the roof ribs nicely. I used the recommended 3M window weld urethane to secure it down. Painted the rim after cutting it out with some primer, and stuck the fan down with butyl tape and screws.
While I had the tools out and urethane open I installed a shore power socket on the side as well. I got a nice heavy marine unit with a stainless steel cover that matches the look nicely on the side.