Here's the template that I'm using on yer truck.
Aerodynamic Streamlining Template
Here's what the template looks like applied to yer current camper top:
Here's what an aero cap would look like on yer truck (look familiar?):
If you apply a Kammback to the camper top, you might see up to the 10% of the template. If you build yer own aero cap, you can see probably up to 35-40% of the template.