If it's on top of the tank, a lot of people just remove the drivers side attachment bolts for the bed and loosen the passengers side. You can (with a helper) tilt the drivers side up to get to the top of the tank. You should also have to remove the filler neck from the bed side.

 

These are the things that can cause that code:

•A disconnected or malfunctioning fuel tank pressure sensor
•A missing, malfunctioning, improperly installed, or loose fuel cap
•A disconnected, damaged, pinched, or blocked EVAP purge line
•A disconnected or damaged EVAP vent hose
•A disconnected, damaged, pinched, or blocked fuel tank vapor line
•A disconnected or malfunctioning EVAP purge solenoid
•A disconnected or malfunctioning EVAP vent solenoid
•An open ignition feed circuit to the EVAP vent or purge solenoid
•A damaged EVAP canister
•A leaking fuel sender assembly O-ring
•A leaking fuel tank or fuel filler neck

There's a 26 step diagnostic in the shop manual and most of it requires specialized equipment. For visual inspections look for very clean areas around vent line connections, especially on the gas tank.

 

Does anyone have a GM Part No. for the EVAP Purge Canister Hose that runs from the 2006 C1500 5.3L Canister Purge Valve (P/N-12581282) to the rear of the engine where it connects to the tubing from the gas tank?