Been chasing P0455, P0449 and P0496 codes for a couple of weeks now on my wife's Tahoe. Also had difficulties in filling gas and a lot of pressure in gas tank.
First got a new gas cap, and got rid of P0455. On Sunday I put in a new EVAP vent valve and the harness and got rid of P0449. Was hoping to also get rid of P0496, but no, so today I changed the purge valve and hopefully finally getting rid of the last code.
Hope that's it for now so I can get it inspected in March. The only thing a bit worrying is that the white plastic clip the vent line connects to the purge valve was missing completely. The line seems to be holding in place, but I guess I need to macgyver something to hold it. Pretty sure you can't find those clips anywhere...