Need Your Help again!! Dimming Lights On Caravan

Just had a week in the lakes using our new car, when we set off all the lights on the caravan worked fine but just before we got to our destination we developed a problem with the lights on the caravan. If I had the main lights on the rear caravan ones went dim and no other lights worked ( indicators or brake lights) When I turned the main lights off the brake lights did work but were dim and indicators were dim, when just the indicators were on they seemed ok. Also when I braked the lights on one of the display panels in the car went off. Once settled at the site I took the car to a garage but they found no fault with the electrics, I know the problem is with the car as I hooked the car up to my in laws caravan who had come with us and the same thing was happening to there caravan. I am thinking a bad earth but before I take the car to another garage for a second opinion I wanted to see if anyone else had any other thoughts on what it might be.