A few weeks ago I replace both factory roof vents with fantastic fans. It took about an hour and a half EACH to do. Not difficult just time consuming, and remember to be careful on the top of the roof. I placed a large piece of wood, positioned over the rivets, to distribute my weight. If you are looking to purchase, check out Vintage Trailer Supply, they came in at about $100 cheaper per unit than my local RV store and the Airstream Store in Spokane. It was worth the wait for shipping time for me to save about $200 in total. There are a number of threads on this task, just use the search feature in one of the blue bars above. Good luck.
1997 25' Safari
2012 Chevy Silverado HD 2500