2 pole or single pole breaker to feed sub panel
It's up to you.  You can use a single pole breaker or double.  Single has potential to be cheaper.

If you don't plan on maxing out the outlet and the RV at once, you could safely go as cheap as a single pole 30A breaker 10-2 wire for the run and a 30 A panel if you can find one that small.

125A panels are pretty mass produced though, so you might find it cheaper ro get a 125A any way.  Just make sure its lugs can safely accept 10 AWG wire and know that you won't be able to use half of the positions in it.

Personally, I'd run 60A or 100A, 240 from a double pole breaker.  Especially if it's going to involve digging, since you can do more later with a full 240v panel and the extra amperage.  Think about EV charging in the next decade.