Back of cab to center of rear axle is the standard that truck manufacturers use when they list the cab to axle measurement of cab/chassis trucks.
The 2 common, industry standard cab to axle lengths for cab/chassis trucks are 60" and 84".
For pickups, the industry standard cab to axle length for longbed pickups (8' bed) is 54".
Short bed pickups (6' to 6-1/2' bed) vary from about 32" to 38", as there is no consistent industry standard for shortbed pickups.
Bed lengths for 60" c/a chassis trucks range from 8' to 9-1/2'. Bed lengths for 84" c/a chassis trucks range from 10-1/2' to 12-1/2'.
Truck campers' center of gravity points are measured from the front lower wall of the camper, rearward. This starting measurement point will be a few inches behind the truck cab when the camper is installed on the truck, because there is a gap between the cab and front of the camper, of anywhere from 3 to 6 inches, typically.
The further forward the camper COG, the better the truck will handle the load.