It's apparently an E-450 (E-Series (formerly Econoline) van cutaway), not an F-450 (SuperDuty chassis-cab). Still, the model numbering corresponds roughly to the truck GVW classification and the E-450 barely makes it into the implied class (4) so about 14,500 pounds is right (the GCWR is much higher, so it belongs in the class) and it's roughly comparable in weight to an F-450.F450 is only rated to ~14,000lb GVWR, iirc. The engines in them were 235HP 7.2L diesels. That's 300kW full power up a steep grade, say 6%, at 60mph.
The E-450 has come with various engines, in the last generation covering just about every engine found in medium-duty trucks. Yes, 235 HP would be common, but that's 175 kW (not 300 kW).
Aside from the erroneous value of 300 kW, I see no reason to assume that any vehicle manufacturer would assume a requirement to maintain 60 MPH up a 6% grade at GVWR. Drive in any mountain area, and you'll find lots of trucks working all-out to maintain much lower speeds up the grades.That increase in grade requires about 840 pounds of force, assuming Ford didn't design for gross towing weight. Assume 30" tires...about 1000ft lb. Back that into a 4.56 rear end ratio, direct drive turns ratio is 219 ft lb. Guess it at 2500 RPM gets us 104HP or about 130kW.
Backing that out of the 300kW leaves us with 170kW used at 60MPH if we assume Ford designd the e/f-450 to climb a 6% grade at highway speed at rated GVW.
An easier way to calculate the power required to climb is to look at the rate of change of potential energy. Energy due to height is the product of weight and height. 60 miles per hour is one mile (5280 feet) per minute or 5280/60 = 88 ft/s; 6% of that multiplied by 14,500 pounds is 76560 lb-ft/s. One horsepower is 550 lb-ft/s, so that's 139 HP, or 104 kW. It's easier (no weird factors to remember) in metric units.
Tire size and axle ratio matter to what combination of torque and speed is needed to drive the shaft, but not the power required.
If the guess of 1 kWh/mile is correct for the energy needed to keep moving against aero and rolling drag, that's 60 kW at one mile per minute, or 164 kW to maintain 60 MPH up a 6% grade... coincidentally about the peak rating the of the example engine.