i have also pondered the problem of a close following trailer and tight turns. Your idea is a good one, so long as trailer weight is low.
it becomes a problem with heavier loads or with absent minded drivers that forget to slide the trailer back.
I think the best solution is some sort of flexible shroud that fills the gap.
Another idea is to move the hitch forward. sorta like this.....
https://video.search.yahoo.com/yhs/s...3&action=click
Many vehicles have roof racks these days making the 5th wheel hitch mount simple. I would think that a lightweight aerodynamic 5th wheel trailer would be practical and might even make highway mileage better.