When you say "trouble" do you mean the GFI immediately and always trips, or just occasionally trips, seemingly at random? The latter is tough to diagnose. I've seen some GFIs occasionally kick off from starting/stopping power tools or appliances
First, the GFI breakers can be quite sensitive. I've had them trip when an extension cord end got wet. (that's what it's supposed to do). So make sure your cordage and connections are dry and in good condition.
...and sometimes, GFI outlets go bad. Is there a different GFI outlet you could try?
If the cords are OK, try to isolate the problem. Start with all the boat's AC load breakers off, anything onboard with an AC plug
- unplug it. If there are still trips, take the Xantrex right out of the circuit and see what you have. Hopefully you can isolate the problem down to a specific device or area.
If the above doesn't help you isolate the problem, an electrician could help you determine whether you have a more serious issue.