I'd like to clear something up about the ELC system on these cars. I spent hours researching it to repair it on my car. I have a schematic somewhere I could dig up if necessary, but I doubt this is your problem.
It depends on the year of the car. On my '99 and I suspect on years with similar body styles (97-05), the ELC (electronic level control) control module located in front of the gas tank on the drivers side, with a linkage to the rear suspension, controls when the compressor or relief valve operates. The compressor is disabled when the ignition is off to prevent battery drain, so it is unlikely this is your problem. This was not always the case on earlier models (pre 97).
When the ignition is off, the ELC control module opens the vent solenoid and releases air from the shocks till the car is level (the car will be lighter from passengers exiting the vehicle). It is possible if your compressor runs continuously for about 3 or 4 minutes when you start the car that there is a problem with the system. The only thing that would cause the battery to drain over night would be if the ELC control module was bad and holding this solenoid on overnight. I remember the solenoid drawing on the order of 400mA if it is undamaged. The ELC has a fail safe built in to prevent the compressor from burning up in the event the shocks leak. If it runs for longer then about 3 or 4 minutes when you start the car it will disable the compressor until the next time the car is started.
As for the alternator, it is not unheard of for a new alternator to be bad. The only way to test this is to remove it from the car and have it tested at your local parts store. Make sure they hook it up right on the test stand. I have watched guys at the parts store hook it up wrong and tell me it is bad, and then I point out they didn't follow the directions.
The only true way to figure out your problem is to pull fuses until the draw on the battery goes away. Last time I had this problem on a vehicle I hooked an amp meter in series with the battery (with the ignition absolutely off) and did exactly this.
I caution you, if the draw on the battery is greater then 10Amps you will blow the fuse in your meter assuming it is a decent meter. If it is a cheep one (ie Harbor Freight) you might just melt it in to a smoking pile of plastic.
Remember these vehicles have fuse boxes in multiple places, including under the rear seat and under the hood.