Welcome aboard. I'm about 600 miles in with the same truck, 'cept it's white, and my highway (improved secondary road with lots of intersections, so stop and go some) was 20.3 in US gallons. I imagine that a freeway constant speed cruise would bring it up some, but like they say not made to be a mileage champ.
I try to look as far ahead as possible and anticipate what traffic's doing. Regular here in the Great White North is about $4.08/US gallon
so it's not low enough yet for me.