If you are connected to a 120VAC 15A/20A receptacle in your garage using a RV30A-15A LONG Adapter it could be your 120VAC receptacle you are plugged into in your garage...
Make sure you are using all three wire connections. Some adapters may have the GROUND PIN removed...
The 120VAC receptacle you are plugged into may not even have the ground wire connected.
Before I hook up I use one of these circuit testers from LOWES and if the test lights don't show the CORRECT sequence I will not use that receptacle - plug it into another receptacle location that tests good.
We are all just plugging away in the dark here...
Roy Ken