I agree that from an aerodynamic standpoint, a short little fairing between van roof and underside of bed isn't going to do much to reduce drag. In the picture I posted above of the 19G, the real aero benefit should come from the aero shape of the cap protrusion itself, not so much how it's sealed to van roof.
As I think about it, air has to go up over motorhome, and/or around sides. The area in question here is probably one of high pressure, but of actually little air flow. I doubt you'd ever see any measurable improvement in fuel economy, if that's a major goal.
If doing this as an aesthetic upgrade, that's a totally different matter.
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