It sounds like you have some sort of drain going that slowly drains the battery when car is off, OR the alternator is weak and not able to keep the battery charged during normal driving (slowly letting it go dead), OR the battery itself is weak/internally shorting or something.
It would seem that any competent dealer could find and resolve this with basic electrical testing. I would ask them which specific test they performed to rule out each of the above possible causes.
Beyond that, for sake of trying something - I would push for a new battery since its cheap and has been a common failure.