<<I seem to remember reading sometime ago, that the heaviest current
drain setting on the fan was the medium (not high or low) settings.
Speed 2 on a 4 speed AC fan (in other words, strangely, the most likely
to be used setting.)>>

The biggest draw on the electrical system is when the AC motor is on the
high speed fan relay which is a full direct drive power from the Vans
battery / alternator.

(This is why the vehcle has a high speed   AC motor relay )

On the lower speeds the power is on the lower speed regulators which
don't tap off as much power from the charging system.