I spent some time tracing out the route. It looks like from the hitch, the feed goes to the front left bottom of the front wardrobe. Here the feed splits to the front lh bottom clearance light, the top front lights, the rh front clearance lights and then down to under the basement where the cable runs across to the rh side middle clearance light. It splits here with one cable running across to the lh middle clearance light and one cable running to the rear rh side that feeds the rear clearance lights and tail lights. I found the fault at the first 4 way split. The feed coming from the hitch had been squashed between the wood framework and the chassis during manufacture. It finally failed causing a short circuit. Fortunately it was in the front lh side of the robe behind the removable corner panel . I could see where the cable had been squashed so was able to fix the fault.