The gas cap is just one of many things that can leak. Hose clamps rust off. Vacuum hoses dry-rot. Valves leak. The easiest way to find leaks in this system is with a smoke machine. It injects a white non-toxic smoke at 2 psi, then you search for where it's seeping out. You might be able to borrow one from an auto parts store that rents or borrows tools.