[hatzie]

Behind the tank is the vent valve. Don't expect this little bugger to last.
The purge valve is on the intake manifold. Make sure you don't have vacuum at the hose pip with the electrical connector disconnected and the engine running.
Gas caps never fix the problem. Don't bother.

Diagnose it.

Query the PCM for the fuel tank pressure with the filler cap removed. If it's not super close to 0"Hg you probably have a failed fuel tank pressure sensor.
Re-install the filler cap.
You should be able to hear the vent valve actuate when you command it closed with the ignition on and the engine off.
You should be able to feel vacuum at the hose connection to the purge valve when you remove the hose and command it open with the engine running. It shouldn't have any vacuum at the hose connection with the electrical connector unplugged. Re-connect the hose and electrical to the purge valve.
Test it further by commanding the Purge valve open and the Vent Valve closed while watching the tank pressure. The tank pressure should drop negative fairly quickly. If the pressure doesn't change you have a really big leak, the vent valve is hung open, or the tank pressure sensor is knackered.

You can change the pressure sensor without dropping the tank by reaching up over the top of the LH frame rail and feeling around for it. It looks similar to a MAP sensor and installs the same way. Get one from a GM dealership don't bother with aftermarket.

Clear the dirt off the top of the tank before you pull the sensor out. After 2003 these trucks don't have a fuel filter if someone didn't install one. Reach up with a blow gun and blast the dirt off the top of the tank.

Unplug the pressure sensor before removing it so you don't damage the connector and wiring.
You can snap the pip on the sensor off so carefully turn and lift on the sensor don't just yank on it.
Wipe some Sil-Glyde on the replacement sensors' gasket so it won't stick to the sender.