Unfortunately, I don't know of an easy way to diagnose this. You really need a couple of good meters.... first, a standard VOM with the capability to measure 10A current via the probes... and second, a clamp-type DC ammeter that you can use on the battery cables to do a quick check for a major current draw. Then you have to check it all the time. When you measure a large current draw at the battery via the clamp-type meter, then you have to start isolating one circuit at a time (pull one fuse at a time, etc - don't forget the external ones for the HVAC blower fan, heated seats, fuel pump, etc) until you can find the circuit which is causing the drain. If it's none of the fused circuits, you have to start checking the unfused items (alternator, starter, possibly the CAN modules, etc).
I have a Fluke 36 AC/DC meter, you can find them on eBay for $50-$100 or so, for example
this one went pretty cheap. Just clamp it around either main battery cable and you can see if there is a major current draw or not (normal should be less than 1 amp, anything over 1-2 amps will kill the battery overnight).
