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I'm the last person to give solid advice on electrical but I had the exact same issue and it turned out to be my voltage drop due to I think 6mm cable running to the rear outlet.
Along with lights for camping it was insufficient cabling. Upgraded to 2 b&s (Dazz confirm?) and happy days.
Could be other things as mentioned above too but maybe take a photo of your wiring and set up, it may help the more knowledgable folks on here.