Electricity is not something to mess around with - one bad electrical hookup and you could fry your {home}'s internal electrical systems. Surge protectors will let you know when a hookup is going to cause problems, and even prevent issues with power surges and outages.
This one is pretty straight-forward. If your RV has a 30A (30 Amp) plug, you'll need a 30A surge protector, if it's 50A, you'll need a 50A :)
You can spend less to get a surge protector that will sacrifice itself if there's a bad surge (meaning you'll be protected, but need to buy a new surge protector); or spend more for an expensive surge protector that is able to be reset after power surges
Internal surge protectors are wired into your system from inside the RV. External surge protectors just plug straight into the pedestal, and you plug your power cord into it. Internal will cost more and be more difficult to install, while external are more easily stolen