Chevy Express 2500 6.0 amps drop to low @ idle

need to drop that 6 foot fuel tank and these links will help Replacing Chevy Express Fuel Pump Nick New Mossberg 500 and Savage Rifle 2001 Chevrolet Express Won Start Fuel Pump Replace

idler or tensioner. check that. Not likely fan clutch or water pump. check PS pump?

amps or volts
11 to 12 amps indicates nothing but if it is volts then it indicates that the alternator is not charging the battery
Battery voltage is 12 -13.5 volts and when charging is 14.5-14.8 volts dc
the rest of your statement indicates that the alternator or voltage regulator is faulty and you should have the charging system checked by an accredited auto electrician

LOW OIL CHECK THE DIPSTICK

could be a vac leak or injector problem

replace the ground stap from the chassis to the engine on the passanger side lower part of engine

Clean the throttle body with throttle body cleaner and make sure the air filter is not dirty or plugged.Also clean the mas air flow sensor with mass air flow sensor cleaner.Give this a try and let me know if it took care of your problems.Hope this helps.Good luck.

454 with ac and a.i.r.

Now change the sulfated batery for a new one.
Vitally

Check power supply to alternator. it might be a blown fuse from the old alternator or you might have purchased another bad alternator.