JC gave us some more info, but WHEN is the fuse blowing? If it only blows when moving, you may have an insulation problem somewhere. If it blows immediately upon starting the tow vehicle, you still could have insulation problem, but maybe a crossed wire.
I found, with a problem I had with the directional signal lights
, that trailers have hot wires going to all the things, turning them on and off by connecting to ground, (I think), because everything had a black wire and black wire in home wiring means hot.
things can be frustrating. They go together fine until you turn on the power and then interesting things can happen. LOL
BTW: Automotive batteries are designed to provide LOTS of amperage. That is how they start the engine. If you didn't have a fuse, there is ample amps to melt a wire at any weak spot along it.