What size wire do I need to run for 10 100 amp Rv hookup with individual breakers off a 400 amp service at 420 feet to the last pole?
You should aim to keep voltage drop under 5% or close to it.  

Given the voltage drop and statutory requirements, you'd be best to run at least 750 or 800 kcmil aluminum wire.  At 320A actual flow, you'll have 3.7% voltage drop for the 240' from the supply to the first RV stand. 

Using copper wire for such a large run is a waste of money, aluminum is fine for feeder.  

You would not be able to splice such large wire to an RV stand directly, so you should come into a distribution panel there, or a little past there.  From there, branch to the various stands.  Each 50A stand must have at least #6 copper or #4 aluminum wire, and I would bump 2 sizes for every 100'.  If you want 2 stands back to back for 100A, try #1 aluminum.   You can run it through a voltage drop calculator to see What wire to use for a particular voltage drop.   The calculator I linked doesn't do very well with 400A circuits, but it's fine in the numbered cable ranges.