My new RV doesn’t have an over stove fan of any sort just a metal barrier to block heat and grease buildup. I like the outside vent that my Freedom Elite had although it had the drawbacks of being another sidewall penetration, it was drafty and had potential insect issues. If we are going to cook an especially messy or smelly meal we often choose to do it outside.
If your vent fan is in your microwave it almost certainly can be adjusted for inside or outside venting and you’d just have to fabricate a duct and wall termination. Parts should be available from an RV supplier. Personally, I wouldn’t bother.
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