2004 Chevy Silverado check engine light

I keep getting a bell and a display reading tighten fuel cap, I had it to the dealer several times for this. They have replaced fuel cap 40.00, replaced fuel neck 360.00, now they say its a vent valve for the emissions, I am disgusted with my dealer and tired of spending money trying to fix this only to get it home and within a few days to a week get the same bell and display. Can you tell me what my problem is and if it is a vent valve where this is located obviously my ASE certified tech at the dealer is not very certified, THANK YOU.