OK, here's the info. First "real" trip was 199.2 miles. The only mod done for the first trip was to lower the hitch (level out the trailer and reduce frontal area).
The drawbar in the first post photo above has a 2" drop, this one has a 4" drop. It needs a touch more, and I have an 8" drop, but that seems a little too much.
The total for the first trip was down 2.2 MPG from normal, but that's not the whole story. The truck is a 4.6L manual trans and generally very capable. By itself, loaded, it holds 75+ MPH on the highway for this route without a problem.
This is Colorado, so there are lots of hills (and satisfying descents), of course, but with the trailer, there were several times climbing it struggled to hold even 65. Certainly added weight is a factor (+900#), but I think the aerodynamic load is killer. (I have a long flat bed trailer I've pulled with a lot more weight and it doesn't have nearly the drag of this sail.)
OK, the understatement is "Go slower." I understand the V^2 contingent -- which is, one of the conflicts because time is also important. Anyway, next step is to try a few aero mods. I've got one in mind that I think you'll like.