You need to be using the 4 prong 240v/120v outlet. Your generator may have a switch labeled something like: 120v only/240v
It will need to be set to the 240v setting.
Don't let anyone tell you that RV's aren't 240v. An RV 50 amp connection is a 240/120v connection.
If you have an adapter to connect your RV 50 amp male into the generator 30A 4 pin and the switch is set right I don't know why you would have a problem except maybe you're overloading the generator. A 6500 watt gen. is only capable of supplying half what an actual 50 amp RV circuit can.