2017 F350 6.7 Super Duty Cooling Fan not spooling up to cool hot engine

[brnewsom]

When towing RV uphill, the engine cooling fan is not spooling up to increase airflow to cool engine. Observing engine oil temps up to 241 degrees, and engine coolant temps up to 223 degrees. I have had to slow down to around 20 mph in some circumstances to prevent engine from reaching derate temp 225 degrees engine coolant temp, and 250 degrees engine oil temp. Dealer unable to find source of problem after 2 years of trying. Anyone else having this issue?