P0442 is for a medium evap system leak, P0455 is for a large leak. There are a lot of things that can cause either code to set. The gas cap seal can sometimes be the problem, or it could also be a cracked or split emissions hose or a bad purge valve or emissions pump. Back in 2010 my Dakota threw a P0442 and it turned out to be a bad evap leak detection pump. I also replaced all of the rubber emissions hoses under the hood while I was at it. Then about 3 weeks ago I had the same P0442 code again. Everything under the hood still checked out OK, so I looked under the truck by the gas tank, the hoses at the charcoal canister are known to crack and set emissions codes. Sure enough, I had a small vent hose with a rubber elbow on it from the canister that is connected to a plastic hose going back to the gas tank. I smeared a good coat of black Permatex on the elbow, cleared the code and have not seen it since.
If you do not see any obvious problems like a cracked hose, it might be best to have a shop do an emissions smoke test. The smoke will show the leak much like when you put a flat tire in water and the bubbles show you where the puncture is located.