Adding a 50a breaker in main panel to feed 30a rv breaker and plug
If you want to wire in 50A for a future, larger RV, then you need to run the cabling with 6/3 UF cable (not NM/Romex, you cannot use Romex because you will be going outdoors).
If you were to do that with e 6/3 cable, with the panel you have bought, that would be fine.
However, it's a common case for someone to buy the wrong thing and then go into backflips doing even more wrong things trying to make the wrong thing work.  Don't bother; get the right thing.  If you are mail ordering this stuff, thats a mistake - local electricians must shop somewhere; shop there.
They make RV panels which have all the breakers and sockets necessary to support all of

a 240V, 50A large RV (with NEMA 14-50 receptacle)
a 120V, 30A small RV (with TT30 receptacle)
typically a 120V/20A plain old outlet for hand tools etc.

This works off a 50A breaker because it is presumed you will only be powering one RV at a time.  But you still need 6/3 outdoor cable; 10/3 won't do.