A gas engine is generally running as optimum as the hardware allows. Many of the chips require premium fuel to get their gains, increasing cost.
You would be better off making intake/exhaust upgrades, a good air filter and headers makes a big difference on flow and economy. My dads 97 454 picked up about 1.5 mpgs from those mods, that's 15%.
-John
2018 Ram 3500-SRW-4x4-Laramie-CCLB-Aisin-Auto Level-5th Wheel Prep-Titan 55 gal tank-B&W RVK3600
2011 Outdoors RV Wind River 275SBS-some minor mods