Pool subpanel circuit correct?
You cannot use any wire smaller than #3 CU 75°C wire feed a 100A subpanel.
You don't indicate wire type you are using for the 50A feed to the RV receptacle, UF cable would require #6 CU, using conduit and THWN would only require #8.
If you wan't to use one leg of the 240v branch circuit to the RV to feed a 120v receptacle you will need to provide overcurrent protection suitable for the receptacle. You can't just put a 15 or 20A receptacle on a 50A breaker.  For the short distance you describe a separate #12 feed might be easier. Easiest would be to buy an RV pedestal already has the breakers and receptacles in place https://amzn.to/3oP1zRQ .