Outside lights fail, no indicators or brake lights

After towing our Swift challenger for about a hour all the outside lights on the van fail, no indicators or brake lights.

The ones on the car are fine, this is a new tow bar and electrics fitted only a few months. We have had it back to have it tested and they say its all working.

We have also had the caravan tested with a full service and they say there's nothing wrong with the van ?

Please help, advice would be good.