IF the run is in conduit the whole way using THHN/THWN conductors, then #8 is borderline OK for the circuit conductors. #6 would give you a full load cushion.
IF there is ANY NM cable in the circuit then you must go with #6 cable.
A #10 ground is all that is required in either case.
I also like the idea of a small RV panel out there.
Something like this will have the two "standard" RV receptacles plus a standard GFI receptacle for convenience. Not bad at $165 either.