With winter grade fuel, you're possibly going to give up an mpg or 2.
From southern Indiana to Ft Walton, where my son was stationed, I'd get 19. But I drove 65. Ran with the flow where needed. Heavier tires and a front end lift cost me an mpg or 2.
A lot of highway mpg depends on the wind. If you catch a tail wind, you'll do great. If you hit a 15 mph headwind, be prepared for a big disappointment.
I had a washer and dryer on back during one of those runs to Florida. Didn't hurt mpg much at all. If I put a motorcycle on back, that really hurts mpg. I didn't know anyone bought a 645RFE after 2013.
Hand calculated, my guess is 16.75. Final answer....