Welcome to the "Club" and the best advise is don't get hung up on the mpg of these trucks as the only way they return better gas mileage is to really baby them by driving slow...
Hard Bed Covers do not help your highway or interstate mpg as the extra energy required to make these speeds simply overwhelms the minor reduction in drag the cover offers. There have been several test that confirmed this, to which I'll state that I added a hard cover to keep out snow and keep my stuff dry the first week of ownership. My general mpg is 17.6 pure highway and interstate...Welcome, I too just purchased a 2016 SV with 6,700 miles. So far I like it. I knew I was going to suffer in the MPG compare to my old econobox. From my reading, you're not going to get a big increase in MPG
Tonneau cover will probably be my first addition to mine though, hopefully it can increase my HWY 1-2 mpg