What year is that truck? It wasn"t long ago that every Chevy/GMC truck out there used the same torsion bars, be it 1/2 ton V6, or 1 ton Duramax.
07 Silverado classic (leftover 06) I had with the 4.3 and automatic got 15-16 on a bad day. Best tank was over 23. Try running your tow mode and keep it at a fairly steady 60-65, and keep your right foot light so it doesn"t downshift.
Kept a book with my fill-ups and miles driven for the first year then just recorded my road trips after that.
Bumper looks pretty good with that bedliner on it.