On the earlier FG-14 trucks (with K series Nissan engine) a truck could go into limp mode due to a defective accelerator pedal position sensor WITHOUT turning on the MIL.
On those trucks, Komatsu had placed the diagnostics connection under the floorboard near the inching cable bell crank. It is a small black, 4 wire connector capped off with a weatherpack cover. You will need an adapter harness to connect it to a standard OBD plug.

The fact that you are unable to initiate the pedal dance to display codes further supports the premise that the accelerator pedal position sensor is faulty.
Without a sound APPS, the ECM does not recognize the command to enter DTC flash code mode.