I leave my field box in my trailer with it's 12v lawn tractor battery in it. I then plug in my 12v Battery Tender to my field box to recharge the field box battery, it's the only battery I charge prior to going to the field.

Agreed, get the proper connections to provide 120vac power to your trailer, a hot male end is asking for trouble. Sooner or later it will be exposed hot and someone will come in contact with it.

Vince