If the gas cap is factory, you might need to Smoke the gas tank and look for a leak with Smoke machine.
But If I have to take a guess and throw a part at it, it will be the ESIM switch on the Charcoal canister, fairly common failure on Chrysler vehicle. and probaly cheaper than the gas cap.
The charcoal canister is attached to the Back of the Gas tank, ESIM switch is the White piece on the canistor, it looks like this.
Note: Not all of them are Colored white, few of them are colored black. And should take you no Longer than 15 minute to replace once you are under the car.
Make sure you jack up the car properly and sit on Jack stand, these Vans are heavy.
Remember to Lubricate the seal between canister and ESIM switch with some grease to help seal better.
To reset the GAS CAP light on the dash, you have to disconnect the battery for few minute, the Code reader does NOT clear the message on the dash.
read this article if you want to know more about it.