P0420 can be a bear, but thanks for a good question with some helpful details.
If you are setting this code at idle, it's 'better' than while driving.
Step 1 is to pull the battery for an hour, and tap the brake before hooking the ECU back up. This should ensure a soft reset and clear the code. A full reset would require an OBDII scan tool, which you might not have access to.
If the code sets again, but still at idle, I would consider replacing the catalytic converter(s). If you have a lot of miles on the truck, they could well be ineffective or clogged. If you have exhaust leaks like a cracked header, it can also cause P0420.
Everyone seems to jump straight to replacing the o2 sensors, but I'd start with the above.