Chevy Duramax Fuel mileage

I have a 2012 denali dually dmax/allison. Love it to death, before i did the full H&S delete w/ 4" dual exhaust i would average 15-17MPG max due to the regens, and about 9-10MPG towing. Did the full delete and no word of a lie got a hand calculated 24.6MPG highway on a night drive with zero wind with the cruise set at 100km/h so that was a picture perfect "ideal situation" but in a real world day to day i get about 18-20, and best ive seen towing is 13mpg towing about 15,500LBS with the boat/camper hooked up. But more often than not iam around 10-12MPG if you include hills etc and thats always set at 90-95 km/h. I personally will never own anything other than a duramax simply because i think its the most heavy duty combination with the duramax/allison, i dont mind paying for reliability. Also i believe the H&S full delete kit will extend the life of the truck substantially, we have it set on the 50HP tune as we have really no need for any extra HP over stock.