engine speed vs road speed + weight + aero drag.
The limo wasn't meant to tow a car... heck the car the limo was converted from wasn't meant to tow a car.
diesel trucks have 2x-3x torque at 1/2 the rpms of that Northstar at peak.
And extra capacity tranny + cooler, +aux oil cooler, bigger brakes, punch a bigger hole for the trailer.... etc.
While Stan may have traveled there in a slower style, he got there and is hopefully enjoying the NC high-country immensely. (the rest of us are just jealous we aren't there too.. )