My mileage isn't as optimistic as some here.
Had a '13 model 6.2 EC/LB, 4x4, 3.73 and my 15 is identical. Average commuting, mostly hiway with tool box and slip tank around 1klb in the bed mileage is about 12.5 off the readout. This is with the weenie 245 factory tires which throw off the speedo a few mph to the low side. Trucks speedo is calculated for the larger tires offered on the up scale models.
Towing heavy low profile flatbed trailer 8-14klb knocks it down to about 9mpg.
Towing 4000lb cargo trailer 8-9mpg.
Put 285s on the 13 truck and that took another 1/2 mpg off but the speedo was reading right with the taller tires.
Never hand calculated mileage on either truck but it doesn't seem far off. It sucks the gas down.
I don't baby it though. You'll never see me hanging out in the right lane doing 55 regardless of what's hooked to the back. If you hyper mile it, you could get a coup,e mpgs better probably.
70mph with a little pop up you might see 10mpg.