Dan, love your marine hatch, and like you I also dislike the standard roof vent approach. For me it is also a question of having limited height in my garage, so whatever I can do to reduce overall height is good.
I am in the Pacific Northwest and will mostly be using the trailer in this area. I am thinking that I can also get by without a roof fan.
Question: what are you doing for vents or fans elsewhere in your trailer? What is your thinking?
Any other builders views on doing without the roof fan?