overflow is before the P trap. Vent is after it. They look similar, and sort of do similar things, but they cannot be mixed.
I don't know if an RV can have an overflow draining to the outside. I do know that an RV tub can have NO overflow at all. Search on tub+"no overflow" in a search engine, and you'll find tubs made solely for the RV market.
So, overflow is connected before the P trap when it exists in the first place. An overflow is an open pipe so it would let insects come in to an RV when piped as you have described.
david





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