GFCI at shore power tripping

I am a professional electrical troubleshooter. A good troubleshooting rule is to go back to the last place you were. So disconnect the heating element completely and try it again.
Let us know the results and we can go from there.
You may call me and I can help you over the phone. Please don’t chase symptoms all over the place because you might accidentally introduce another fault and then you really won’t know what happened.