I don't think the elec fan/maf voltage is an issue - unless your battery/alt/charging circuit is having problems. The battery is there to provide the voltage the system needs when the alternator can't keep up due to electrical loads like big fans. Of course, over time, if the alternator isn't recharging the battery, that could cause other system problems - but you'd be having starting issues if that were the problem. If you suspect your alternator isn't keeping up, that's something that needs to be remedied - but I don't think it's causing your bad gas mileage. If it started after a swap from 24# injectors to a different maf calibration/30# injectors - I'd suspect the maf calibration.