My first thought is a dead short between the + positive terminal and the - negative terminal.A multi-meter on resistence will tell you very quick if it is a dead short, disconect from battery both wires. the meter will read no resistance, between the 2, also check between the positive + wire hot terminal and you rig ground.
Here again if this leed is shorted the problem is more then likely in that wire, the netural terminal should read 0 to the trailer frame = ground -. Then go inside to the 12v panel disconnect both + & _ wires and read them the same way. If you burnt both terminals & wire, it is not a loose connection.I am not a know it all but 55yrs as an electrician and 2 degrees in elec eng give me some insite to elecricity.
Take care.
