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P0442 is a medium sized leak in the evaporative emissions system. This can be a hole in a hose as small as .020", not always easy to find.
The gas cap is a definite possibility. Start by making sure it's tight. REmove it and look at the seal, and look at the check valve in the center. If you see any signs of rust or corrosion then the cap is leaking and will need to be replaced.
From there you would want to do some very close visual inspection of the system hoses. Anything from the filler neck hose to the canister, and then on up to the engine can have a leak. The most common leak on these trucks is directly on top of the evap canister. There is a line that goes across the truck from the tank to the canister, and then an elbow hose where it goes into the canister. This is the common spot for leaks. You may need to remove it to see the cracking.
An internally leaking purge solenoid is possible, but they gave almost no problems on these trucks at all.
If you can't find the leak by eye you can take it to a shop and they will have a machine for finding these leaks. Since so much of an evap system can be obscured from sight leaks can be very hard to find. The machine for finding them hooks up to the evap system and pressurizes it with smoke. The smoke will leak out wherever the evap leak is so it can be found easier.