I did initially think it could be the plugs, but I saw that they were replaced 20k miles ago. However, that was back in 2014. I never heard of spark plugs having an "expiration date".When i visit my parents in Socal i go up and down the grapevine... In my GX (285s/Metal Bumper + Winch/ Armor/lift) it struggles when im in D however if i get out of overdrive its fine... My average going from Bay area to Socal is about 17 mpg cruising at 70mph GPS (65mph dash).. above that the wind gets pretty harsh on the MPG.
Check your tire pressure, clean your MAF if you havent yet... maybe check the plugs if they are still in good condition.
I'll also look at the MAF.