I found the latch on Ebay, but it's only the latch, so I'll hang out here to see if someone can explain how to get rid of that sliding plastic thing and put one of these in with a new piece to hold it in place.
Model X19H Expandable
GMC Sierra Pick up
Been a camper since I was a kid, and that was a long time ago :)
From pup tents with no floors, to tents with floors and flies, to a pop up, to the expandable.