Tom-
I also have a GFI in my garage that trips most of the time when I occasionally plug my MH into it when I begin loading. In addition to your refrigerator, your inverter/charger begins pulling load and probably some other smaller loads. I have to run a longer extension cord from an outlet in my shop that is not on a GFI, and everything works fine. I recently had to change another GFI that has a garage refrigerator, sprinkler system, and fountain pump running off of it. It would trip frequently and I could never figure why, but after replacement, everything works fine. Bottom line, the GFI you're using may need replaced also.