I'm assuming this is a Sunline Tran-Sport and not a custom made hauler, right?
That story you got from the repair company is crazy... The window won't be cheap, and shipping especially won't be cheap, but it shouldn't be 1200.
Remove the entire window and frame from the coach. On the outside of the frame (which would have been up against the wall and hidden), there will be dimensions and model number info from the manufacturer of the window, which is probably Kinro.
What you can do is call Kinro. I believe Kinro won't sell to the public, you will probably have to go through an outside source, but explain your situation first and that you don't have a manufacturer to go to, because that's probably where they'll want to send you.
Also, a little tip. Since Kinro ships right out of their warehouse, make sure to request extra packaging all the way around from Kinro, regardless of who you order it from. These windows are very easily bent/damaged on the edges when transported in freight.
Kinro can be contacted at (574) 264-6590 or email@example.com
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR