I would suggest getting a volt meter hooked to a cigarette lighter adapter so that you can monitor the voltage as you drive. Should be around 13.5V with the engine running. If the voltage is good, try disconnecting the battery each night. If it holds a charge then the problem is likely a voltage drain.

One trick that has worked for me in the past for isolating a battery drain is to remove one fuse at a time in the dark. I was able to locate the offending circuit by seeing a small spark jump the gap at the fuse as I removed it.
Worth a try