Summary:
Maxing out the payload caused a 7% loss in range, but he didn't account for aerodynamics - the load was sticking out well above the roof of the cab. For reference, aerodynamic drag caused by crossbars mounted on the roof (with NO load on them) can easily lead to a 10% range decrease, so apparently the full payload had no discernible effect on the R1T range.
Like
@sciencegeek says, he should have done this test with an empty container vs. with a full container because then the aerodynamic drag of the empty and aerodynamic drag of the full would have been the same so we would have gotten a real answer based on payload alone. Also, like
@sciencegeek says, once you're up to speed and not accelerating/decelerating that extra mass, the payload really doesn't matter - range in "city" driving will be affected way more than in "highway" driving because of all the stop and go.