if you are using a drop cord find another cord and give that a try then take the tester that all are talking about and plug that into your gfi first then check to see if there are 2 yellow lights lit up on the tester if there is plug your cord back into the outlet and check the female end of the cord doing the same test if 2 yellow lights again then plug trailer back in and check all outlets in trailer with tester looking for the same 2 yellow lights I am guessing that you have a bad ground somewhere if you do all this and still have the issue then I would recommend hiring a electrician to do some more in depth tests