Yes, you can use it. I've never yet seen a 240v hot tub circuit that didn't have a neutral. But, verify there's a white wire in there along with the ground and 2 hots.
Are you installing a 30A or a 50A RV outlet? If a 30 amp then you'll connect to ONE hot wire and the neutral. If a 50A you'll use BOTH hot wires and the neutral. In both cases you will, or course, also use the ground. So yes, a 50A RV outlet does infact have 240v despite what some people will lead you to believe.
I would get an electrician to at least check your work. If you get it wrong you could destroy some or all of the electronics in your RV.
