A new roof air is generally NOT installed in a fan opening, as there is insufficient bracing present to hold the 100+ lbs. of the roof unit. Many folks he use Carrier low profile units, and the factory just started using them on the Safari Sport line. They also use Dometic Penguins thru the entire line of Airstreams.
If you have your interior skins off, you should see a 14" X 14" re-inforced opening in the area above the axels that will accomodate your roof unit. You also need to run the 120VAC power fed (12AWG) and a 12VDC power line as well for the control voltage. A drain tube is also a must to keep the condensate from draining onto your aluminum and staining it.
I have installed many of these units, and I prefer the Dometic Penguins for their outstanding warranty service and tech support. Carrier makes a nice product, but I have had major issues with them concerning warranty coverage. YMMV!