Do other 12 volt
things work in the trailer? Like the lights?
Aside from chewed wires and a blown fuse, there are a couple of other possibilities. The old fans themselves may have gone bad, or there may be corrosion in the fuse box that is keeping current from flowing to the circuit.
If you do find that the wiring from the fuse panel to the fans is bad, and you decide to run surface wiring, be sure to disconnect the old wire at the fuse panel.
There are surface wiring conduits you can pick up at your local hardware store, Menard's, Home Depot, Lowe's, etc. Some are flexible and will conform to the curved walls of the trailer. I did a Google search for "surface mount wiring" and found this online store:
Cable Raceway: Surface, Wall & Floor - Cable Management