2000 dodge ram 3500 24v, Batteries are being drained, bought new batteries a month ago and they are still being drained. Already checked the obvious. Help?

 

I had the same thing happen, I recently had back surgery and was in the hospital for 4 days and got home didnt even start my truck for 3 weeks, went to get in it the other day and dead as a door nail, I had just put new batteries in and thought oh crap the alternator is bad but looked in the cab and had a phone charger plugged in the power plug in the center console, unpluged it had my wife get the batteries out I cant lift anything yet so bless her heart she had to do it, charged them fired right up. It set for another two weeks and had no problems starting. I now that the cell phone charger ran the batteries down, I think it sounds far fetched but I beleive this was the cause. So I dont just leave the phone charger plugged in all the time now.

 

After the obvious, the next place I would check is the starter solenoid contacts. They develop carbon and then leak voltage to the fuel heater and grid heater solenoids, not enough to trip them, but enough to flatten the batteries overnight.

Another place that is always leaking to ground is the hot wire on the tow coupler. What happens there is corrosion builds up inside the housing, and sends current to ground at a low rate.

If the draw is pulling the batteries down quickly, then unplug your fuel heater and the intake heater grids and see if that solves the problem. If it does, it is the relay pack for those, and generally includes bad starter solenoid contacts at the same time.

Corrosion in the brake light switch is a common voltage leak, but that is an obvious one that you likely already checked, as it the multifunction switch and the headlight switch.

If you have a bad relay, you will need to pull each one from the PDC and do a continuity check on each to find the guy with a shorted contacts. The relays in the PDC hardly ever fail except on trucks where the owners wash the engines.

 

I had a short under the driver's seat on one of the motors and it was pulling enough to make the power seat fuse too hot to touch. Not enough to blow the fuse, but enough to zap the batteries.

 

Not sure if 2000 had them but quite a few folks have had trouble with faulty seat belt modules draining their batteries.
If you purchase a factory service manual it will have several pages on how to trace down IOD (ignition off draw) problems.
Personally I don't see how you could test without one unless you have tons of time on your hands.