I see a theme here... leading edges.
This has been mentioned here MANY times, but once more doesn't hurt:
https://history.nasa.gov/monograph46.pdf
Within it you'll find not only the often mentioned "Dryden van", but also the image below (p. 28-29). A 30% improvement due to the leading edge treatment!
I often post in truck/trailer/RV posts. If you search my username, you'll find LOTS of posts about the subject.
With the pop top, you have done one of the best things you can do aerodynamically, reduced the frontal area.