First new window in place. Without trim rings rv window are cheap and impossible to install in a conventional rv. 16 gauge steel makes it "no problem"
The aluminum flange is too thin to effectively countersink panhead screws, so stainless fasteners backed with trim washers on the outside.
The trim washer spreads the load, and on final install holds a little slug of sealant to make the fastener hole water tight. Just run a bead of sealant or butyl around under the window frame to the exterior to bond the two together and make it weather resistant.
On the inside, foam insulation, with whatever you want for a trim ring since it doesn't need to be structural. Since the vehicle wall is 3"+ thick, there aren't any trim rings anyway.
Anyway I am happy with the way it is secured and the installation process is straightforward.