Something is draining your battery thats for sure.. disconnect 1 battery terminal and use a multimeter to get a current reading with the ignition off. This will tell you how much current you are using with the ignition off.. not sure on what a 'good' reading would be but suspect under 3 amps
If that checks out fine then chances are its your alternator
Have you measured battery terminal voltage with the engine running? If it reads 12.5v then check the alternator output
terminal 'b' (+ve); measure the brown insulated terminal with 10mm nut to earth.. Should be 13.0 - 15.9v if not, its shagged
