I wouldn't complain about that mileage with a G56 AND 4.10's! I get 15.5-16.5 on average mix of driving. All highway all 55 mph has yielded me a best of 18.3 i believe. Once. I can get low 17's if I go 60-65 everywhere, all highway.
I have 3.73's, single rear wheels, short bed, 285's. You sure you have 4.10's? I run a good 2125+ at 70mph with the 285's.
'06 Ram QCSB 2500 G56, Valair DD, crazy larry, ride-rite bags, EV2's-lift,rail,boost,EGT, 20/7/2 mic. fuel filters