You can test the purge solenoid and vent valve by removing them and applying 12 volts and ground. You should hear the solenoid click, and you should be able to blow air through them when they’re open, and should not be able to blow air through them when closed. If I remember correctly, the purge valve is open when powered, and the vent valve is closed when power applied. Someone can correct me if I’m wrong.
I went through an evap system problem with my 05 Sierra. I had a different code though for a gross evap leak (P0455?, don’t recall for sure). I determined the purge valve solenoid was failed open. I replaced it with a Duralast “lifetime warranty” part (my bad), and the check engine light came on again in a few weeks. Upon testing with 12 volts like I mentioned earlier, the new sensor had failed. I replaced with oem and it’s been fine now for months.
Also, I was also able to take the charcoal canister apart and clean and inspect it. I’m not sure if this can be done on OP’s model. I also blew air through the lines with the solenoid and vent valve removed to make sure there were no obstructions. I used human air, not compressed 
. Good luck.