TankK1500

So, recently bought a 90 K1500. The previous owner told me that he had battery draining issues, and would take the negative cable off the battery every night. Well, I forgot to, in all my happiness after I replaced the spider gears in the rear end, and got it driving. Went to start it yesterday, hooked it up to my car to jump it, and it would only crank over very slowly, so I got out my charger/jump box, and when I went to turn the key, all the relays(practically heard like all of them) started clicking, the radio started acting up, and the A/C control unit was flashing. All of it reminded me of electrical interference you see in the movies. I know the previous owner stated it didn't start doing the battery draining issue until after he re-installed the radio, but it doesn't stay on after the key is off, and the batter was carrying 12.7 volts, with the charger on, and using a voltmeter, and unhooked from the truck. I checked the battery today, I put it in the back of my car, and its holding at 12.5 volts. The charger is a schumacher charger, and it was showing an SUL code on the screen, which according to the book, that means the battery is "sulfinated?" Also, i noticed the ground wire for the battery is fixed to a bolt on the intake? Just seemed weird to me, because most grounds are to the frame or body bolt. Any ideas are helpful. Thanks in advance.