Here is an interesting read about alternators and mpg. http://www.metrompg.com/posts/alternator-optional.htm
I messed up my calculation earlier. For some reason i was thinking 1 hp was 1500 watts. So if my stereo draws a full 800 watts that is really over 1hp of energy required, at a cost of about 2 hp engine mechanical power. That is nothing to sneeze at. You might need 20 hp to keep your truck going 60 mph, tire size tread and psi, ride height, aerodynamics, and all engine accesories make this number vary a lot. I would guess an automotive AC compressor's mechanical load is another 3-5 hp. If your typical cruising speed is 45 mph or less, the HP needed to maintain speed is quite low and the percentage of fuel used for A/C and alternator loads is much higher.
If i am wrong, HHO generators would work. But they don't, since they take a lot of direct current and the alternator load cancels any benefits of burning the HHO. Another gimmick that would work if i was wrong: Electric superchargers.
Sorry to see this thread go off topic. However, a 6 cylinder truck is about MPG and any way to increase it = more money in your pocket.