1990 Chevy Astro new battery keeps on getting drained from

It's not the battery. I replaced the new one already after a week. It's not the battery. The alternator is only 2 and a half years old. The battery cables are about 6 months old. The first time, it was my fault; I left the lights on. I then had the battery replaced. It was running good. I always watch the battery symbol gauge, and it wasn't going down; it stays in the middle. However, I do usually only go out at night, and I usually have the heater and radio on, but again, the battery gauge thing doesn't show a drain from that. Then, the next night, (it has been getting colder lately) when I start, it doesn't crank. (I did not leave anything on.) I just hear a click, and then after a few tries, no click at all. I buy a battery charger and charge it. After 30 percent charged, it starts up, and I properly remove the battery charger; I leave it in idle for a half an hour. The next night, I start it up, and it cranks poorly but doesn't start. After a few tries, it no longer cranks; it just clicks, and then just a click. I would think that after idling it for a half an hour, it would fully charge. Am I wrong to think that? And even if I do find out that it is still not fully charged, what caused it not to crank? Please help.