Welcome to the forum.
Our previous coach, a 2015 Challenger 37ND, had 2 emergency exits and one of them, the big one in the bathroom, was hinged on top and had a bar to latch the window open and it came with a screen.
Hehr manufactured the windows so you might try calling them to see if they have a retrofit kit for your window.
In the pic below, the red U shaped handle is used to remove the screen for exit.
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