I'm having similar issues and this is a fairly recent post so I figured I'd throw my hat in the ring here rather than starting ANOTHER P0456 post! lol!
2012 Tradesman, 4.7L, 4x4, Rambox, Flexfuel, no mods
I've had this truck since new in 2012 (maybe late 2011?). I've still only got 70k due to short commute 4 days a week and wife doesn't like my driving.
Recently had evap system fault code and now "GASCAP" on the odometer at startup with the little pictogram of the gascap. I replaced the cap with a cheapo from the local parts store. Still faulted a couple weeks of use later. After filling the tank I had a puddle of gas in the driveway that caused me a little uptick in my concern meter. I crawled around under but couldn't see exactly where it came from. I dropped the tank and found a crack where the fuel line comes out of the pump hat. $350 later I've got a new fuel pump in there but still have the codes, no puddle though, but that was a one-time thing so far. After a little Youtube University I find the white sensor box on the back of the charcoal canister could be the problem. I pull it out and sure enough
clackity-clack when you shake it and it had a big crack and corroded wires at the sensor side of the plug. New sensor on order.
Replaced the sensor today, cleared the codes with a reader, drove around a bit, turned it on and off about ten times. Still has the GASCAP odo code but no check engine light indicating a code. After a bit of reading it seems this has to maybe be at a certain level of full in the tank and a certain amount of engine cycles before this clears, even if it is actually already fixed? How many cycles? How long? What conditions must be met? My cheapo code reader can't initiate the evap test it needs to pass to quit spitting the code at me....