Boost kills gas mileage. Period! I don't think there's any "false advertising" going on with mileage. Many of us DO get around 17 in the city. My last tank was 18. If you're down to 12, it's either your driving habits, your geography, weather, or there may be an issue with your truck. I never got that bad even brand new with no miles. Also, if you're in "sport" with AWD engaged, that'll kill your mileage too. I ran sport and AWD and that tank (city) was down around 14-15. And you have to hand calculate. My cluster is higher than hand math.
Hopefully, you didn't buy this thing expecting "good" gas mileage. Again, it's a 6000 pound brick. FWIW, my 668 hp V8 Blackwing is surprisingly more fuel efficient than my Expy is. I figured that would suck on gas, but that'll get 20+ aound town driving hard. Highway it'll easily get 25-27.