I'm right there with you. Sea Level in Southern California and will leave Trip B to monitor the miles between each oil change. 16+ average for me too running 87 Octane. I've seen 20+ on a 500 mile road trip when the truck was new and I was babying it, but when driving it like you want to on long trips I'll usually drag it down into the 18-19 range. I have no plans to try Premium fuel.
High octane for tunes and towing. My truck is a car to me. Don't race it may never even tow with it. 87 will be good for me. I feel these trucks should get 20 MPG all day on long trips if not to many hills. I got average 22 MPG hiway 5 and crappy hiway 97 in Oregon 1100 mi trip.