I agree with Baldy. It sounds like a charging issue to me.
When you wind up the engine, does the ammeter show a charging state? It's not unusual for an ammeter to show a "barely charging" state on idle.
When you have the engine running, does the ammeter show a closer to center reading than when you have the truck on but not started?
~#~#~#~#~
I don't think that you've fried the battery, because you're getting a negative reading on the ammeter -- which means that whatever you're running is drawing from the battery and not your generator/regulator setup.
I can't speak for the rest of the system until I've got some more information.
