Check Engine Light Keeps Coming On

I own a 2004 Honda Odessey, purchased about 5 mos ago at 35,000 miles. Within the first week after purchase, the check engine light came on. Took it in, was told it was the loose gas cap code. We never saw the cap loose, but have made certain after that 1st occurance to crank the thing three full turns (not just clicks) every time we fill it. The check engine light has come on 4 times now, the latest being this last week. Every other time we took it in... they continue to tell us it is the gas cap code and that we aren't tightening it enough, which is just ridiculous. We have driven the car hundreds of miles after the code comes on, refilled tank, cooled it down, etc., it never goes off on its own. We KNOW we are tightening the cap. I guess I'll be taking it in again this week, but I'd like the real problem to come to the surface. Anyone have any ideas or suggestions about how to proceed?

Evap codes are a real pain in the ass to diagnose. Its like trying to find a pin hole in a mess of tudes and pipe. Usually on a car with a problem that cannot be fixed with regular troubleshooting its helpfull to have a leak tester. it basically injects smoke into the system so the leak is detected.

I assume the gas cap is a Honda cap?