Hondahound,
Something you might want to try is the engine load at idle with all accessories off then try it with blower fan, defrost, low beams highbeams, turnsignals, brake lights etc. This may give you an indication of alternator load and how that affects idle and driving fuel consumption. To check residual load you could unplug your alternator.
I'm curious and my van can't have a scanguage
I'm running on deep cycle batteries to eliminate these drains. I have not done the testing to see how much benefit I get but MetroMPG has
Test: Alternator vs. no alternator = 10% MPG gain @ 70 km/h
Good luck.