The best I ever was able to accomplish in 1 sitting was an entire large Domino's pizza followed by a 1/2 gallon of Eddy's cherry ice cream. I would guess that meal was somewhere in between 6000-7000 cals. I was bloated as fuck for 6 hours. The dump I took the next day was epic!
I've eaten entire economy size boxes of junk cereal too. Lucky Charms is my fav.