rv

[enireh]

1955 rv came with two prong cord works fine unless I hook up cord with a ground then gfci outlet trips without ground gfci doesn't trip

 

[GoldDigger]

1955 rv came with two prong cord works fine unless I hook up cord with a ground then gfci outlet trips without ground gfci doesn't trip

Either you have dangerous leakage to ground in some of the rv equipment or there is a connection between neutral and ground inside the RV.
Which is it? An ohmmeter will tell (with the shore power disconnected!)