Recently I came across this video:
Now they did say it did have some improvement at the end. But notice how they used a hard tonneau cover in their test run. They never used a camper top.
But they also said that driving with a pickup topper did NOT hurt mileage. No Tailgate, Tailgate up and pickup topper were about the same!
But who knew that using a tailgate mesh would get the best mileage!