My LMM does the same thing. I have replaced my sending unit already (warranty) to try and fix it but did nothing. My DIC is accurate to the amount of fuel burned. I have a 26 gallon tank too and just run it till it shows I have used 22 or 23 gallons of fuel. Works for me that way. GM should have either fixed the issue or allowed for larger tanks on a diesel if they wanted to be that conservative with fuel. At least the new LML trucks come with 36 gallon on the shortbed now.
2008 GMC 2500HD LMM SLT CC SB 4x4 Z71
Buckstop front bumper, WARN M12000 winch, PIAA 520 ATP driving / PIAA 510 fog, Westin Platinum Series nerf bars, Pace Edwards Bedlocker tonneau cover, Cognito UCA leveling kit and HD Idler Pivot Assembly, Cognito Pitman and Idler steering braces, Bilstein 5100's, LT285/70R17 BFG AT KO tires, 17 x 9 XD Series Crank Wheels, 15% Tint Driver/Passenger windows, GM Tow Mirrors, DDM Tuning HID lights 6000K 35W