Drained batteries

Hello, I have a 1994 Hyundai Sonata 2.0L with umm to many kilometers, I have actually stopped looking couse it is well over the 400,000 km mark! My car has been a great car, very reliable, interior is slowly falling apart exspcially the power windows, and sometimes my clock will dim out, or my gauges will 'flat line'( no reading of rpms, speed, gas etc) Anyhow to get the the point. My last battery had died on me several times, and was weak since. A week ago it would no longer hold a charge. I cleaned the posts checked its connectivity etc. And it still turned out I needed a new battery. With the new battery installed everything worked great! For a week. Now I am right back where I started with a drained battery not holding it's charge. I can't think of what would cause a brand new battery to drain like that. Unless my alternator isn't working anymore. I need advice!