The tub on the Caribou is a bit more complex than you have drawn. The rear of the camper is nearly as wide as the opening of the full-sized truck bed. I have measured our Caribou 8 (I think the tub is the same shape for full-sized trucks, just 8 feet long, not 6.5 feet: 1500, 2500HD and 3500):
-our camper tub's rear width at the truck bed tailgate is 58 3/4 inches
-our A dimension (tub section between wheel-wells) is 48 3/4 inches
-our B dimension (the tub rise to top of wheel-well) is 13 5/8 inches
*remember: because the tub of the Caribou series campers is a T shape at the rear, the camper tub dimension will differ from behind the rear wheel-wells to the truck's tailgate between various models of short-bed trucks vis-avis various model long bed trucks!
S-