Electrics problem

Burtspieeater

Came back today on the Dieppe - Newhaven Ferry. We loaded early and so the car/caravan was on the ship for around 6 hrs. We were at the front of one of the queues but disastrously when I came to start the engine the battery was flat  Luckily
the crew were excellent and within 5mins the battery was started with a battery pack

I can only presume that, as I had left the caravan attached, the caravan (fridge?) had somehow drained the car battery. How do I check the relay is working?

Please explain it as if you were talking to a 5 yr old as electrics is not my strong point