Brian,
Sorry that you are still frustrated.
I confirm that eLearn diagram E7010 for the key code system, shows the LED in the instrument panel as being supplied via fuse F37, which is sixth from LHS on the bottom row of the LH cab fusebox. I also have my own home brewed computer diagram, which shows F37 as a red 10A fuse in that location and that it supplies the instrument panel LEDs.
My thinkng was that if the code receiver (not theECU) was losing supply due to a wiring fault between it and F18, the LED would illuminate, which it should as the code receiver would not be working. You seem to have confirmed this.
When the problem occurs, do we know whether this is because there is no supply to the code receiver due to a wiring fault, or because the code receiver is not recognising the key? My suggestion of a temporary LED indicator to monitor the code receiver supply at the code receiver, was a possible way to resolve this. I fully appreciate that making the necessary connection close to the code receiver, is not the simplest of tasks. To elaborare, if the suggested LED is connected, and it remains illuminated when the fault occurs, then a supply failure to the code receiver from F18 is eliminated. That leaves a wiring problem to the ECU from the code receiver output, and a faulty code receiver as possible causes.