I have always felt that a full camper shell provided better aerodynamics when pulling a trailer - kind of keeps the airflow up and over. But nothing other than gut hunch and seat of the pants comparisons to back that up. It felt to me like there was less drag on the trailer with the shell on, when I was pulling into a headwind. For this reason, as well as storage space, I will probably put a full shell on my new truck even though I prefer the looks of the hard tonneau covers.
I do know that studies have shown that tonneau covers are the only thing that gives any reduction in drag on a bare truck (with no trailer), and that was only about 5%.
2012 Flying Cloud 23-D
1971 Safari 23 (sold)