Help Please? Power cord trips GFI

Thank you. We took your suggestions and went back to storage to trouble shoot. We turned off house/engine batteries, all the breakers above driver seat (including inverter). Plugged in and still popped the GFI 120 outlet at our parking space. We spoke to the on site mechanic and he said some of the larger/older coaches are not GFI friendly. Also the facility where we park have about 100 coaches plugged in and said depending on coach may or may not work because of all the other coaches plugged in. He suggested an echo charger for both house & engine batteries. He seemed knowledgable however it seems odd to me that it would trip when all is off. Also, we live in AZ and the weather is sunny and a beautiful 68 degrees outside. I wouldn't think weather/moisture would be the issue. But what do I know...that's why I am on this forum. ;-p