Article 551 has been discussed in a few of theses posts.
A minimum of two RV spaces are required to use Article 551 for site wiring.
The feeder to the RV panel is required to be sized for the expected loads. That doesn't necessarily mean a full 50 amps, but it doesn't necessarily mean 30 amps is sufficient either. What is the actual load of the RV and what is the actual load of the equipment you wish to connect to the 20 amp receptacle?
By installing the new RV panel you are changing this circuit from a branch circuit to a feeder. That means you will need to install two ground rods at the new panel. That's the impact not using Article 551 has on you. 551 does not require ground rods, but not using 551 requires them to be added.