Why does my truck start on its own, with no key, when battery leads are attached?

Initial specs: 2003 Ford Diesel F-250 Super Duty 7.0
2 batteries, 2 solenoids.

This all started out because it wouldn’t start, and had to be continually jumped off. Batteries checked out good, but a terminal was corroded through, so I replaced that. Batteries still wouldn’t hold a charge. Realized I didn’t reattach alternator lead to new battery terminal. Batteries charged now but next day it wouldn’t start again. Took off the starter and took it to autozone and they said it checked out, when I hooked it back up and connected the grounds to the batteries it started turning over the engine.

No key is in. Even with the key I get no power inside the cab even with the leads attached, except that the starter goes nuts trying to turn the engine over. It won’t start though, even when I try to jump it off and let the starter do its thing. Now one battery (the one closest to the starter) bubbles water when I connect the negative and gives a bad popgun or firecracker smell when it sparks.

Any ideas or hints? (Please excuse the long explanation, I’ve been wrestling with this for almost a week and left out quite a bit of the drama between me, the truck, and Autozone)