My 98 K1500 with the same drivetrain and rear gears as yours but with stock 265/75/16 tires get 14-15 mpg on average with 17 being the best it ever got on a long trip over 20 years ago.
I also have a 98 K3500 with the 7.4L/ 4L80E/4.10 gears/ 245/75/16 tires and it gets around 10-13 mpg unloaded and 8-9mpg hauling or towing.
Yep, with stock sized tires, 15 sounds right.
The 7.4 BBC is a drinker but the torque for its time was fantastic. I drove a K3500 CCLB 7.4 SRW a few times and although it was slow as a dog, it felt like you could pull a house off its foundation. Paled in comparison to the 6.5 TD of its time I'm sure but