"A few weeks ago the battery started slowly dieing while it sat. Eventually it would not start if it sat more than a few hours. I replaced the battery, and things were good for a while. Then the same thing happened again. Now the battery won't charge at all. I replaced the alternator. It worked for a day, then the same thing started up. I jump it, it runs fine, then as soon as I shut it off it's dead."
Try to install a extra ground wire (25mm2) from the engine to the chassi & one extra ground wire (6mm2) from the alternator to the engine.