I installed a 50A OEM receptacle (Marinco) in front left corner....run #6 wire to a 'manual transfer' panel (originally had an auto transfer switch but they were faulty Parallex models)
I can plug in OEM 50A power cord front or rear OR plug in 4KW 30A generator into either receptacle (have a 50A Marinco plug on my gen power cord...appropriately wired 30A)
Sounds like you have a sub-panel with 3 circuit breakers.
How are they tied into RVs AC System?
Need to ID circuits in order to understand how to wire gen output.
Post some pictures of this wire box etc.