Very sorry to hear of your loss.
A fuse will only blow when the current drawn exceeds its rated limit, usually caused by a short circuit to earth. Two possible causes of fire, insulation on wires rubbing through and exposing conductors; this can happen if wiring goes through hole in bodywork without a proper grommet. Another possibility is high resistance joint, where trailer wiring is teed in, this could overheat without fuse blowing. If the cause of the fire can be proved to be due to poor workmanship of towbar fitter then there might be a chance of claiming from his insurance.
Don’t accept first offer from insurance company, argue a good case for them to pay what you need.