In relation to the problem the OP has, If I have understood it correctly the power supply apparently changes polarity al by its self with no intervention by the user. I presume the OP is looking at the caravans Polarity indicator, as there is usualy nothing in the house that would give such an indication.
I doubt the polarity in the house changes, its more likely a fault in the caravan.
The fact the OP has found it necessary to ask this question suggests they do not have electrical qualifications so, either way its a job for a professional.