Don't know how electrical savy you are but get a digital or analog (meter with a needle that moves) volt/OHM/current meter that can read about 10 amps of current. I have an old analog Simpson 260 that I use. Disconnect the positive terminals of both batteries. Be sure you have selected the AMP function on the meter. Place one lead on the battery positive terminal and the other lead on the wire removed from the battery post. Do this with only 1 battery connected. The meter should read a positive current drain. If negative, reverse the leads. There should be a small current drain (around 50 to 100 milliamps) indicated. If more, then you have found your current drain problem. Start pulling fuses one at a time until the meter goes almost to 0. A lot of times when people install after market electronics, they connect them directly to the battery vice going through the ignition switch. That will definitely run the battery down.

DSMMH