Jeff - It can get confusing, especially if you don't own, or have worked on, RV sites before. Luckly, I have done both.
Large RVs will require a 50Amp connection to operate everything in the unit; however since many campsites might not have the 50Amp service available, all the loads inside the RV are 120V. That way the RV owner can connection his 50Amp plug into a 30Amp receptacle (via the proper adapter) and can still operate the RV unit, just limiting his usage. (For example, many large units have two HVAC systems. The RV owner would be limited to running one HVAC unit at a time, or risk tripping the 30Amp breaker that his has connected to.)
Similarly, you can get a 20Amp adapter for your 30Amp plug and do the same thing. (I own a RV with a 30Amp service.) Once again, you must limit the load. (I can run my HVAC on a 20Amp circuit, but I have to turn everything else in my RV off to keep from tripping the main breaker.)
If you are installing the 50Amp receptacle, you must still use a standard double pole breaker in the electrical panel. And yes, for a 50Amp RV, you will want to install two phase conductors, a neutral, and a grounding conductor. Everything will work just fine. You will have to install the two phase conductors on opposite phases to prevent overloading the neutral conductor.
Conclusion: The only reason RVs do not utilize the 240 volts for any of their loads is because sometimes the RV owner will use an adapter to connect to lower amperage power sources.