Kill everything at the converter, disconnect the batteries and isolate their leads if necessary. Plug it back in to your house. If it trips again, I would plug in the shore power cable without the TT plugged in to the other side...... that should give you a good idea where the problem is after you get through those steps.
One other thing, not to insult, but how well do you know that breaker circuit? Sometimes the garage circuit is tied to several outside receptacles as well as receptacles in one or more bathrooms. This is done to meet code for GFCI requirements and save money by putting the GFCI at the "top" of the circuit. The cheap skates that built my house did it that way.....it sucks
