No, I agree with you Callan, the Cackler's story is largely a sad one. He's in there for black comic relief, with the emphasis on "black." (Actually, I added him late in the writing process, after about the third draft, because I wanted more dialogue in the beginning). Your interpretation of the Cackler is quite interesting. Like I said, I haven't figured out his whole back story, but I'd love to have an excuse to rant more in a fictional setting.
I haven't read the book in a while, but as I recall, he was fed by Eric and Hernando. But the way I imagined his survival, he's like an old-school Cynic who blends in with the chaos because he thinks of himself as an animal. Of course, that wouldn't suffice to save him from the zombies, so there's got to be more to it. An animal would have much less pride than a person, though, so the Cackler could resort to ugly means of surviving, sort of like the Jews who hid in outhouses during the Holocaust.
It's a little awkward commenting on the book since you haven't read it all yet, but there's another, more major character later on who likewise thinks of himself as somewhat less than human.