High engine load causes more cylinder heat generated, lower speed means less air cooling, when you combine the two you are testing the limits of the cooling system.
E-fans or belt fans are fine for cooling rads in good shape, but nothing beats air speed when driving at 60MPH, lol.
Even a good rad will allow some above normal heating if towing a heavy load up a long grade, i.e. lower speed/higher load.
But it won't "over" heat, just run warmer that "normal".
If you had a belt fan I would suspect bad fan clutch first, but with e-fan I would say rad problem.
You do have a fan shroud with the e-fan, right?
The shroud allows for the full cooling potential of the rad, without it only a part of the rad is getting the full air flow of the fan.
The high load/low speed also heats up automatic trans fluid, which adds to the rads cooling load if it is also the trans cooler.
You can do the "Flip Test" on a rad after it is removed.
With radiator out, put cap and drain plug back on.
Place rad so cooling tubes run up and down
Block lower hose outlet with your hand
Fill rad with water using upper hose outlet
Stop filling when full
Remove hand from lower outlet
Water should exit quickly, upper and lower openings are same size so air won't slow it down.
after water stops flowing tilt rad a little to get the rest of the water out.
Now the test
Flip rad upside down
No water should come out
If any water comes out it could only come from blocked cooling tubes.
You can often lay rad flat and fill it with CLR and warm water solution, let it sit 20 minutes then flush, repeat as you see fit, and this will get some flow back.
But once the build up starts it is just best to bite the bullet and get new rad, and change coolant every two years to prevent future build up.
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