[Ed Wright]

Adger, the hand held tuners you buy only alter the timing at WOT, or very high loads (like zero manifold vacuum) and usually only from about 1600 RPM & up. No part throttle timing tweaks, which is where you are towing that trailer down the highway. Timing is backed down at part throttle to lower HC emissions. That engine, and the 454" were tough to get past EPA certification.
Hand held tuners usually do lean WOT A/F also, since when they finally go into Power Enrichment, they are too rich from the factory. The throttle angle for P.E. mode, after the 60 second delay those & 454" come with, is set low enough that towing a heavy trailer in a head wind, or long grade, they will go into P.E. mode, meaning running down the road with an A/F ratio in the low 12-1 range. Mileage killer. Also, the Cat cool down mode is a mileage killer. When the ECU calculates the car's must be hot, it goes very rich to cool them down. It goes into that mode often when towing. Mileage killer.
If someone removed the cats, that needs shut off to save fuel. I just had a customer I picked up his 8100 truck 2 mpg driving, 4 mpg towing his travel trailer. Someone had removed his cats. :-) Those cool down parameters can be altered to improve mileage, when cats are still there.