If the entire window frame is missing then your only hope is a custom RV window manufacturer like
ALL-RITE"
All-Rite builds AFTERMARKET RV parts. We are unable to cross-reference by year, make, model, part # or VIN #. All products are built by ROUGH HOLE sizes. The rough hole is the hole left in the coach after the original part (door, window, etc) has been removed. Measurements are taken from wood to wood for both height and width.
All-Rite Custom Manufacturing has a wide range of RV windows for sale. We custom-design insulated windows with removable fiberglass screens, and picture windows in single or dual panes. Our catalog also includes slider windows built to fit your rough hole size (including T-slider, vertical, and horizontal) and torque windows with single or dual openers.
In addition to manufacturing high-quality RV windows, All-Rite also specializes in creating custom replacement window screens. Our screens are made to order, and they work with any of All-Rite’s windows as well as a variety of other brands."
They do also carry a variety of window styles and sizes, you can check their catalog.
They can take your dimensions and make a window to your opening size.
But, you might need to be sitting down before getting prices as they were not cheap before all the supply issues we have now.. And their website right now is indicating production of the awning style you have has been temporarily suspended so you may need to consider replacing with a different style of window if you want to close up the hole.
On edit..
In Elkart Indiana which is where a lot of the RV manufacturers are based out of is a variety of places that buy up surplus RV inventory when manufacturers change models. Most of these places are not on the Internet and you will need to go there and search their warehouses. A friend of mine often takes camping trips out there to find "upgrades" on the cheap. I did order some stuff from one that did have an Internet presence but they are no longer in business.
HERE is a Yellow pages search for Elkhart Ind..