I had a 2013, now have a 2015, both w/5.3 & AFM. No engine problems on either. Also, mine does not "regularly" get 20-22. I drive very sensibly & probably avg 18 mpg overall. These are 4wd trucks, but I always select 2wd mode unless there is a problem with snow. Using synthetic oil.
I agree that it's not very often at all that I see the truck in the "V4" mode. The '15 model did recently have a left front wheel bearing go out, which surprised me in this day & age. I realize this is not NAT approved, but I have the extended warranties so the bearing deal didn't cost much. They replaced the hub also.