Adding 50 amp service is not just a wire and a receptacle. If you want 50 amp service to be an addition to the original 30 amp service, you'll need a separate load center, circuit breakers, a power inlet, 6 ga wire to connect inside, and a 50 amp cord. I'd suggest to search for 50 amp cords before proceeding, the price may deter you from choosing this route.
One of the better ways to access additional power is to install a separate 20 amp circuit,using a cheaper power inlet or shore cord and connect that into the typical pedestal 120V receptacle along with your 30 amp shore cord. You can dedicate this additional circuit to whatever appliance you wish.