I have a similar problem, same converter.
I installed a Morningstar PureSine 300w inverter, and hooked per install sheet. 120v Neutral is bounded to Frame (Bat-) "outside" the inverter. Then to a gfci outlet.
To do a test I put all breaker to off and try to backfeed the panel trough an outlet, but it trips the gfi... I unbounded the neutral at the inverter and it does not trip... and was able to put on a few breaker even run the fridge...
I removed it, I dunno how safe it his...
EDIT !!
I Found my issue, I was still plugged in my house.
Even if the breaker inside the trailer was off, it still provided a path from neutral to ground from the house panel.
Now no problem with my GFI, I'm not gonna use my inverter when plugged in!!
Edit
I used my Multimeter and put it on the "beep" when it finds a path, Opened the trailer panel and checked the Neutral Buss-Bar and Ground Buss-bar.
Make sure every power source is unpluged and troubleshoot by removing some wire, one at a time till it stop beeping