01, Grand Cherokee, vapor leak

01, Grand Cherokee, 4.0 L, 160K. The check engine light keeps comming on and the dealership says its a vapor leak. They run thier tests and shut off the light and say that its fine. Light is off for a day and then comes back on. After 2 replaced gas caps and 4 computer checks they still can't figure out the problem. Smoke tests on the fuel system brought up nothing. Next step is removing the gas tank and digging deeper. Is that reasonable? Can't imagine its cheap. What can I do?