Faulty 12v supply ona caravan?

Hi I have a lunar clubman 360/2 approx 1992 model. I can't get my water pump to work, nor the inside roof lights. If I run off of battery, water heater and outside light work, but sockets do not. Sockets only work if I have the caravan plugged up, but watar pump still does not, or inside lights. Anyone have advice or can guide me to help? Cheers.