When using RV generator to power house in emergency GFI trips in RV?

I shut off main in house and all breakers. Then start (4.4 kw)generator and plug male to male wire from RV outside receptical to house outside receptical. When I turn 1 breaker on in the house, the RV inside GFI circuit trips..... BUT when I do the same with a portable generator (3kw) it works just fine for running lights and the fridge? WHAT IS WRONG?