I'm a bit confused about the 4 wire receptacle. Is it wired to allow your generator to supply power to the house, input to the house or does it supply power out for other uses, generator? It can't do both. If it does feed the house from the generator, it would have to have a fool proof manual or automatic transfer switch to prevent your generator from backfeeding the neighbourhood and potential harming the lineman fixing the problem on the pole.
If it is wired correctly for 220, one leg on each side different phase of your panel, there would be 20 amp available on each leg, depends on the RV how much on each leg could be operated on 20 amps.
I would say, just just use the 30 amp receptacle that you have installed and let them use the 30/50.
Ken