I hate to agree with you, because people will start doubting me, but my RSTD has an incredibly bad aerodynamic profile. On the other hand, when it rains I stay dry. If I can keep it under 60, I get really good mpg, especially if I am not running 10% ethanol. Good luck on either of those, though.
On your second point, I ran in 4th gear almost all the way from Lubbock to Albuquerque. I was towing the trailer and wanted to stick to the powerband. I didn't keep track of the exacts, but I got better mileage than I did riding from Houston to Lubbock using the 5th gear.
After that, I stayed in 4th unless I was cruising down hill at a good clip.
As a rough comparo, before when I traveled 70 mph with the trailer, I got about 90 miles before my low fuel indicator came on. Running in mainly 4th gear, I got 100 miles, and even 120 miles once (when I was in slow sections coincidentally without ethanol). The better economy was noticeable.
(BTW, on the 80 mph section of I-10, I got 80 miles before I saw the low fuel warning.)
Dave