No, the alternator has not been replaced but when you put a voltage meter on the batteries when running it shows over 12 volts charging but with key turned off and using a volt meter you can see both batteries go dead in about half an hour.
What is parasitic draw? All vehicles draw some power from the batteries when the car is shut off. Certain accessories such as clocks, radios remote door openers and alarms always need power. The normal power used is called parasitic draw. It is always a very low draw so it doesn't run the battery down. If you install accessories like shortwave radios or plug things like invertors and portable refrigerators into the accessory socket and operate them too long without the engine running you can drain your batteries to the point where the engine won't start. If there is an electrical short in the vehicle or a malfunctioning accessory it could be drawing much more than its normal load causing a drain on the batteries. If you leave your door open and the inside lights stay on all night you could drain the battery down till the car won't start. My 98 draws 3.5 amps with the front door open. Usually what happens is the truck is dead when you first go to start it.
The first thing to do is a test for a major short. Remove the positive and negative cables from the batteries. Put an Ohm-meter across the positive and negative cable. If your reading is close to 0 Ohms then you have a direct short. I read 150 ohms on my stock 98. You need to trace the short before you can perform the following tests.
In order to check for parasitic draw, you need to be careful so you don't ruin your meter. At the risk of sounding like an elementary teacher, here's what you need to do.
Measuring Current Draw
It took awhile to figure it all out over several YEARS. I kept replacing batteries, with the problem much worse in the winter than summer, when it was rainy than dry.
I had several shorts in my truck.
When you do a parasitic draw test in a dual battery system and you have an electric winch, you should rewire the truck temporarily as two separate circuits, one to the winch and one to the truck. Each should be checked independently from the other.
If you are drawing too much current in the truck, my first steps would be to check engine compartment components. Disconnect the hot wire to the alternator to rule out diode backdraw in the alternator itself. Then, disconnect the main engine cable connection to the electrical block.
Disconnect all add-on components wired directly to the battery or positive main engine block terminals. Look for elimination of the excessive amperage draw, one item at a time. Disconnect the starter motor, check the draw.
Once the engine compartment is ruled out, I would go to the fuse box under the dashboard and disconnect all add-on components wired to the fuse box or extra hot leads (one always hot, one ignition controlled hot) that AMG supplies for add-ons. Do this one at a time, looking for correction of your excessive current draw.
Finally, I would pull all fuses one at a time.
In my case I had corrosion on the winch hand controller connection that allowed for a constant low current flow. Even the wires to the winch were blackened inside their insulation. The winch itself never activated spontaneously. I moved that connection far away from the winch and away from salt spray from the road.
I had a short in the shifter that ultimately caused a short to ground and blew fuses when I turned on the lights which activates the panel lights. I replaced the shifter (obviously, more to this story).
I had a large aftermarket hot wire to the amplifier (connected to the battery with a fuse) that rubbed the insulator coat off and grounded to the body when it was wet or metal of the body contracted in the cold and completed the circuit.