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.
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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.
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What I would do is use a portable Honda 30001 type generator with a 30AMP RV Receptacle wired like this pictorial
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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..
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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