Did it come with a battery originally? If so, it was probably a group 24 12V which would be fine for something that size. The positive wire goes from the converter to the battery, the negative from the battery goes to a frame ground and the negative from the converter meets it there.
2013 F-150 EcoBoost MaxTow
2013 Jay Flight 28BHS Elite
"My wheel lugs require more torque than your Honda makes!"