From an efficiency point of view, having it before the cord is fractionally better, but probably not sufficiently so to worry about too much. Just be aware that you're trading current for voltage, so for a given power consumption the current carried by your cord and the campground outlet is higher with the autotransformer than the current going through the RV. Don't skimp on the cord.
From a keeping ownership of the autotransformer point of view, having it next to your RV is probably better, especially if you can have it inside the RV or chained securely to it.