like bush pilot mentioned
kill everything, pull all the fuses, disconnect the batteries
**** Important do not reconnect both batteries yet*****
then connect just the ground on the main battery (drivers side).
get a good voltmeter and connect the black lead to the positve batt terminal and the red lead to the disconnected red battery wire.
now install one fuse at a time and check the voltmeter for movement (draw) if that circuit is pulling a load the voltmeter will move move. if it doesnt move, then insert another one, one at a time checking each time until you find the one or ones causing the draw. when you find one pulling a load, make note of it and then remove it and continue on to the next fuse, remember if the door or hood is open and you do not disconnect the courtesy light switch it will pull a load.
when you have checked all of em then go back and one by one look at each circuit that cuased the load, and see if its a problem or a light load.
also if you have everything unplugged and off ... when you connect the voltmeter it should not move, if it does you have a short somewhere. could be small or big. we will need to hook up an amp meter at next class time.
but if it was me with 3 year old batteries, i would change em first.
they could be grounding out internally.