If you dont have one/two already, I'd get at least one wide-band. I've used Innovate motorsports programmable* LC1 before to replace upstream narrow band sensors and also data log. What this will help even without ECM tuning is determining exactly where open loop is AND because its a heavy truck, where COT (catalytic over temp) is. If you have access to tuning software you can find out your target AFR... hint it almost certainly is not 14.7:1. In 2003-2009(?) NGC computers it is something like ~14.4. Moving that up will increase MPG, note the LC1 can do that even if you cannot tune the PCM.

Its been a few years, I have *no idea* about 6000lb truck tuning but the scheme on about every vehicle under heavy load that i had a wide-band in will eventually start dumping fuel to cool off the cat and keep it from melting. I don't know what wizardy they do with your gen computer these days but I'd imagine a heavy truck gets into openloop quite often and when towing probably can get into COT often. You can see this on a large hill with a wide-band 02 sensor output where your cruising at part throttle closed loop at the target AFR, then as throttle increases (TPS v) the computer will switch to open loop tables and there will be a noticeable drop down to 12-13AFR for a period of time. With enough load for long enough and then BAM suddenly drop AFR to 10:1 or richer (COT) until you let off the enough on the throttle. Knowing those points that can get you a couple MPG just by changing your foot habits. I suspect you are getting into open loop a bunch and thats were the tune will help.