I am assuming you are looking for a portable 120VAC power source to power-up a 50AMP RV.
The 50AMP service for RV's is actually two ZONES of 120VAC at 50AMP each. Consider this pictorial from Google Search showing a typical shore power wiring for a RV 50AMP service receptacle.
You will need a 12000 WATT generator to produce two zones (or two legs) of 50AMP service. This will not be a portable type generator.
What I would do is use a portable Honda 30001 type generator with a 30AMP RV Receptacle wired like this pictorial
Then I would use a RV50AMP FEMALE to RV30AMP MALE ADAPTER that is wired like this to plug the 50AMP Trailer shore Power cable into..
This will provide 120VAC wired to both 120VAC 50AMP zones in the trailer and will power up only a total of 30AMPS of appliances.
Just me thinking out loud here...
Roy Ken