p0449 CEL

[08z71]

my question is i have a 2008 silverado z71 5.3 with a nelson 92 octane tune with 25k miles, and last weekend i decided to put arco premium in my truck instead of the usual shell premium, well weds morn my truck threw the solenoid evap leak code p0449 and i went to the dealership and they couldnt find nothing wrong and gave me a new gas cap, so this morning on the way to work the same code came back.
my question is would the arco gas have something to do with it since i only use shell could i just maybe buy some octane boost and burn the half tank i got left and maybe be ok or should i go back to the dealership and tell em to figure it out, i know this is a common problem
also if i have the nelson tune in the truck when i go to the dealership would that be an issue?
thanks
Kyle