Welcome to the forum.
As for "cutting a 8.5x3" hole in the "center" of the rear wall on a Keystone trailer, I have not yet encountered a rear wall on any Keystone with a FILON rear hanging panel that doesn't have either a 2x2 wood or a 2x2 aluminum stud "dead center" on the rear wall. The exception to that is floorplans with a large rear window on the rear wall....
So, do some careful measuring and some "very detailed exploring" before you start cutting the rear wall.
Now, if your current range hood is "slightly off-center" then you should be able to avoid that "center stud"... And, yes, there are several brands of "RV Venting Range Hoods" on the market. Most any RV dealership, Amazon and EBay all have access to get what you need.
Here's one 20" rear ducted range hood with a 12 volt fan and light:
https://www.amazon.com/Black-Rear-Du...3-786e4cdc8f10
You'll also need a square adapter duct extension and the outside "flap closure vent"....