I have been away and not seen your post so you may be already sorted. However if not I would suggest going to a local auto electrician, preferably one with MH experience. Failing that an approved Motorhome/Caravan repair workshop locally.
Your issue could be with the camera, the 12v live feed to the camera, the video wires between the camera and Tomtom, the tomtom unit itself. I had a similar problem some while ago where I found that the video function of the in dash multimedia player was faulty. I bought a clip over video display for my rear view mirror (about £35) and rerouted the video leads from the back of the dash unit up to the new unit. Now I have a continuous rear view while driving and reversing, located in a far better position than the middle of the dash.
Hope that helps and you get sorted. Electrical problems are notoriously difficult to diagnose without physically having the vehicle and suitable test equipment.
Good luck in getting it sorted.