Something seems wrong if you are getting 16.5 mpg on the highway. I just took an 800 mile round trip to see the total eclipse. My path took me on I-68 and I-79 for about 40% of the trip. These interstates have severe grades. I ran about 75 mph on average on the limited access highways and around 45-50 mph on country roads. My total average mpg was 18.8 as I pulled into the driveway upon returning. There is one thing to be aware of when traveling on highways which is using cruise control going up grades absolutely kills gas mileage. If I set cruise and let it control the accelerator when going up grades, I am lucky to get 17 mpg. The problem is the cruise control is much too aggressive with downshifting, which guzzles gas. I find that when going uphill, putting my foot on the accelerator and pushing it down a little more than the cruise control does, I can keep it from downshifting a lot of the time. Then if it does downshift it doesn't go from 6th gear to 4th but to 5th. You'll get a feel for how to fineness the throttle over time. I can do this without turning off the cruise control. Just doing this should get you an appreciable increase in gas mileage if you travel on hilly roadways while using cruise control. The more you can keep the transmission from downshifting the better your mpg will be.
With all the above said, there are things like worn spark plugs, weak coil(s), bad O2 sensors etc. that can lower mpg. Have you checked for any codes being tripped? This might give you some valuable information and a direction to follow to improve mpg. I run about 2k rpm at 70 mph so it seems that you are running in 6th gear at this speed. Keep in mind that Explorers and Mountaineers are basically 5k lb bricks moving through the air so they don't coast the most efficiently.
