A 5er rides the displaced airstream of the truck, unlike a TT which has to move the air again. Similar to a canopy helping get better economy than one without for the same trailer. They are higher, but only one air disruption vs two.
I thought that was pretty common knowledge? 5ers of the same weight/length always seem to do better on fuel than their TT counterparts.
It's similar to how an extended length shank will have a noticeable decrease on economy. It's all about airflow.
Personally I have a friend that would get the same mileage towing a 22' TT vs his previous 30' 5er with a 2nd trailer behind it. The TT was approx 4Klbs, and the 5er with extra trailer was probably 3x as much weight, if not more.