I would start with a draw test , seems that something is drawing enough to kill the battery. You will need a multi-meter, set it on Milli amps and hook it between the positive battery terminal and the positive cable. Wait for 5 minutes(average time for system to settle) and if there is more than 50 Milli-amp draw you have a problem. You can start with pulling the cig lighter fuse(most common) and keep pulling fuses and relays until you see the meter drop below 50 milliamp. The key needs to be off and all doors shut and if there is a light under the hood disconnect, no extra draws. we can diag more after we have isolated the draw.