I just had this too and ended up finding two leaks. The fist was a small crack in the evap hose right by the battery terminals. Temp fix was some electrical tape and perm fix was to sleeve it with some transmission line hose for the autoparts store. Now i though i had the problem licked but it changed to a p0456 which is a small leak and i was still getting the gascap message on the dash at start up. it ended up after many more hours of troubleshooting to be the gasket on the charcoal canister where the main lines and pump fit in. $5 for the dealer and a 5 min fix. Now runs like a champ with no codes.

Here is what it looks like MOPAR 52129436AA - MOPAR® Vapor Canister Seal for 07-14 Jeep® Wrangler & Wrangler Unlimited JK - Quadratec

Dave