The Vista/Sunstar LX series from Winnebago had a similar window, you might be able to purchase from them or at least get a part #.
You might also have good luck with these guys, not related to Winnie but have a large selection of parts. https://www.winnebagoparts.com/
Afterthought - I did a little playing with their online part finder, I think its this one: 184878-03-CHT "Window - ASM-Stat". I'm at work so I can't install the online parts viewer to actually verify that part #, but hopefully that gives you a starting
Worst case, call Winnieparts. When I was on the hunt for something they were able to run it down pretty quickly.