Caravan earthing / door frame shock

I have a Elddis 1997 Jetstream ex300 and have just noticed that when I am connected to 240v hookup while at home I am getting a tingling coming off of the door frame.  When I turn the power off it goes. 

I have searched and seen many different responses but not being technically minded I just need to know, in layman's terms, what the problem is.

Is it the caravan earth that's not making a good connection?  if so, where is it earthed? is it to the Chassis or somewhere else?

I am a bit worried about this as I have children and just need a quick and simple fix (hopefully) as have not got the spare cash to take it in to have it looked at professionally. Obviously if it's major, then have no choice.

Thanks for reading