I began by removing the Jalousie window sashes, wooden jamb, and then cut away the aluminum fins all the way around the window. The exterior side at the top had a hinge that ran the width of the window. From the inside I sliced the hinge from end-to-end, removing the articulated half that attached to the glass. I used a Dremel tool at the ends where the corners were too tight for my grinder to fit. I left the remaining piece of the exterior hinge in place which served as a typical Z-bar or window flashing.
Cutting the flange
I used a file to clean up the edges; an easy task with the relatively soft aluminum.
Here's a step I should've photographed more thoroughly. Because the new window was 3/4" thicker than the original I had to attach an additional piece of aluminum to the window where it would protrude inside past the original frame. I used a piece of 1" x 1/4" channel, cut to the window height, and attached it with rivets, in keeping with the original Airstream assembly techniques.
After removing the sash from the new window, I insert the frame. I had measured it close enough that the frame fit snugly with just enough room for minor corrections if the rough opening (which wasn't really rough) was out of square. I removed the top screw on the jamb side of the awning hardware.
The original awning hardware screw
I drilled through the hole where the original screw was through to the original window frame. I can't remember what size drill bit I used, but it was big enough for a #8 screw to thread into tightly. Check out my huge Popeye forearm. 
I replaced the original small screw with a #8 x 1" Stainless Steel screw that went through the new window jamb and into the original frame. I did this on both sides, at the highest and lowest screws on the awning hardware.
Near the bottom of the window I drilled another hole for the #8 screw.
I tapered the hole with a countersink.
The tapered hole.By the way: I wouldn't bother trying to level the window as you would in a typical house installation, since the trailer itself may not be level. However, I would try to make it square. Also, you may want to put a couple of additional screws at the top, but they need to be much longer, because there is a larger space between the new and existing frames. Do not put screws in the bottom, as this increases your chances for leaks. The weight of the sash hangs on the side frames. The top and bottom bare no weight.
I inserting the sash.Both the sash and frame get masked, for clean caulking lines.
I can't remember exactly what caulking I used, but it is from the Sonoplast family. I used the aluminum color, of course. I caulked the entire perimeter, leaving a small weep hole at the bottom left and right corners (which, because of the angled wall, may not work).
The tape has been carefully removed while the caulking is still a little wet (after about 1/2 hour). A beautiful, insulated, dual-glazed window with modern awning hardware, that works great!The window is an IWC 21-1/4" x 39" Aluminum Block Frame Dual Glazed Low-E Glass Awning Window, approx $300 plus installation.
