Wiring fault?

We have a Ford Iverco horse box. We have just taken it for its plating and found that the horn did not work, we took it the garage that did the mot and they found that a relay had gone. However this has left us with only the microwave and television working inside but we have no power to the water pump, loo and lights. We have checked the fuses and they seem fine - we are at a total loss as what can be wrong. We need the box for a show for this coming weekend.