I found my vertical sliders on eBay. They fit between the ribs without having to cut anything but the skin.
As previously mentioned, RV windows have drain holes for their proper orientation, so you probably don't want to install a horizontal RV window in a vertical orientation, only to realize later that there's water coming in or that gravity makes them slam shut or open. (Vertical sliders have one or two stops for staying open.)
For what it's worth.
Best wishes.

My goal...
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...is to live in a trailer.