If you're wiring a 30A RV socket, it needs to be 120V; 240V there will harm the RV systems. Far too frequently professional electricians get this one wrong as the 30A RV socket looks superficially similar to the older three wire 30A clothes dryer receptacles, which are 240V service.
If you're wiring a 50A RV socket, as others have said, it's exactly the same thing as a 50A electric range socket (among other uses) and is indeed a 120/240V split phase socket. The 240V won't harm the RV because it isn't wired such that 120V loads are across the two hots. (Most RVs have no 240V loads, so everything connected to the AC circuitry would be a 120V load.)
If you're wiring a pre-assembled campground box with 50A, 30A, and 20A sockets, it's a 240V circuit and the box is pre-built to split out the 120V circuits properly.