You will be plesantly surprised by the hiway milege. My 07 Classic LBZ gets around 12-14 in town but on a hiway trip (no trailer) that will increase to around 21-23. This truck really likes to run at hiway speeds.
2007 Silverado Classic 2500HD, EC, SB, 4x4 LBZ, Allison 1000 6 speed, Banks 3" Intercooler Tube, BD LH Exhaust Manifold, Diamod Eye Downpipe, MBRP 4" Cat Delete Pipe, Banks 4" Monster Exhaust, GoodYear 265/75R 16E, Eagle Alloy 16"x8" wheels, Autometer EGT/Boost gauges, 50 gal in bed aux fuel tank.
A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed.