Grew up in Washington and Alaska, I only ever heard it called a camper shell until I moved to the East Coast. Everyone here calls them caps. When I started doing research people here thought I was looking for one of the big campers you see on old pickups.
Over here in Aus, we're with the Canadians, it's called a canopy. Unless it's built on top of a flatbed with gull wing side doors and draws, then it's called either a service body ( for tradesmen) or a camping body, - same thing but used for camping rather than work.