I have to agree with YC1. I have a cheap analog plug-in voltmeter I got from CW and it reads really high when my batteries are fully charged and the inverter is on. 130+. Measuring it with a good Fluke multi meter the voltage is right around 120V. Also, when I plug the same analog meter into a Pure Sine Wave inverter, it reads correctly.
If the GCFI outlet is constantly tripping, then either there is something with some kind of ground fault plugged in to one of the outlets that GCFI plug supplies or the outlet is bad. Check all the inverter powered outlets - don't forget the TV and refer ice maker outlets. Remove anything plugged in and see what happens. If it still trips, replace the GCFI. They do go bad.
34' Holdiay Rambler Neptune
Solar and Jeep