2005 Chevy Silverado P0446 Dealership cannot fix

My dealership cannot fix my problem, I keep getting p0446 code, dealer replaced vent valve twice, replaced the evap canister, basically tore the whole system apart and could find no faults. I did mention that it seems like it only comes on when it rains as to which I was told there was no way that rain was causing my problem. If anyone on here has any idea for me, Id greatly appreciate it.

after the vent valve replacement does the system work code free for awhile? repeatted failures of those valves are normally caused by repeat flooding of the cannister by overfilling the fuel tank. when it clicks it's full. as well alot get stuck by undercoating guns. A smoke test been done? Did this code start after a fuel pump or tank repair? what's the evap pressure sensor reading? will the valve actuate with a scan tool?

The vent valve was replaced twice, leak test was done, wiring was checked. I only fill the tank until the pump stops. I have driven 100 miles and it comes on, or 1200 miles b4 it comes on, it is always associated with rain. When the light comes on, at my next stop I shut the truck off and check the gas cap. There is always a lot of vacuum right after the light comes on leading me to believe that the vent valve is not operating correctly. The dealer felt the same way, but could not reproduce the problem at their shop. Could it be the rain possible shorting it out? Due to its location, I can see how it would be hard for this to be possible.
Thanks

Oh yeah, no work was done prior to this problem arising

Gm trucks used to have alot of issues with corrosion in the fuel pump wiring along the frame, I'd be rechecking there for any issues like green corrosion, loose pins, poor grounds etc. That would make sense with your rain scenario and the problem seems too intermittent to be yet another valve. I'd mix up a salt water solution and spray it along the harness and connectors going to the valve and the pump and see if the problem doesn't become more common. Might be worth trying another dealer at this point as well if they're stumped.

Thanks for the input, I attempted to take my truck to another dealer and was told that due to the fact that it was an existing problem that was being handled by another dealership, they couldnt touch it because the warranty wouldnt cover it until the problem was resolved by the original dealer.
Thanks again for the input, I will check these items and update if necessary

That sounds like them not wanting to take it on! The truck still has warranty left? If so then keep on them to fix it!