2016 will not stay in 8th gear + poor mileage

I drive highways a LOT for work. Whether I'm doing 80 or 65, the truck just does not wanna stay in 8th, and only stays about 20% of the time when doing 80 if I'm lucky, on very flat roads or downhill. When I'm doing 65 it is a lot better, but SOON as I start going up a hill, or need to give it any gas, immediately to 7th. I've read about this issue on 2015 trucks, and some say they got a new flash and it helped, any word on 2016's? I've had my truck almost 3 months and have 7k miles on it already. It is lifted 2.8" front and 1.5" back, and has Dick Cepek Fun Country's that are 66lbs each. I know this setup doesn't help obviously, but I wouldn't think it would make this big of a difference.

As far as economy...I'm only getting 18.5 average according to Fuelly, and I drive highway probably 80% of the time. My tires are 1.2" taller than stock, much heavier, on factory 20's, with a small lift, so of course all those don't help either, but when I do around 65 average I can pull off 22 or 23mpg. My evic is fairly accurate actually with the speedometer being off. Can't truly get accurate numbers due to the change in tire size, but I know it's close. I do use the cruise control, and have tested that my foot gets better mpg, but as much as I drive, I am lazy haha.

Any input would be helpful, thanks.