[aerohead]

Unfortunately, we don't have any back-to-back performance comparisons between a box-cavity and boat-tail of equal length.
These nested backwards facing steps would be 'way out ahead of nothing'.
Since we never see them on nuclear submarines, airships, commercial airliners, bullet trains, etc., there's an implication of inferior performance compared to a proper tail.
Unlike a Class-8 tractor-trailer van, loading and unloading from the rear of an RV is a non-event.
If you need to go up and down the ladder, any tail could be hinged at one side, and swung away if needed at the campsite.
If your time and finances are liberal, you may want to start with a 'low-ball' project like a cavity, with no expectation of 'terrific' results.
If you only want to build something 'once', and then I advocate for the most complex boat-tail you have the courage of taking on.
These projects can constitute up to 800-man-hours if you want something with an OEM look to it.