Is the white wire from the GFCI breaker attached to the neutral bus? I know it's an obvious thing to check, but I feel it needs to be asked. Other than that, there must be a place where there is either a short to ground. With the main (30A) panel breaker OFF, use a multimeter to determine if there's a short to ground via a possible error in wiring. That's all I can think of at the moment. Also, with out actually being there to see the install and inspect the wiring, it's kind of hard to make other suggestions. Others may have better luck than I.