Check the current draw with no load and the batteries fully charged. Then shed some loads like the amplifiers and see where the big change is occurring. You can check current by adding an inline load resistor at the battery and measuring the voltage across it where I=V/R. There shouldn't be much current to start with and your just locating the source of the drain. It is hard to use voltage change as a means to find source drains.

I did some troubleshooting last night on my Blazer and it wasn't much fun.