The answer depends on the generator.. SOME do have two coils, and a switch, and a 30 amp 120 volt twist lock and a 30 amp 240 volt twist lock.
Some do not
However this applies.. IF you try to connect both legs together on the 240 volt (4 prong) Twist lock.. You may not like the result, which can range from a tripped circuit breaker to a destroyed generator..
But if you have a 30 amp 120 volt twist lock.. Just set the switch to 120 and let the generator do the work for you.
You can get the proper Generator to RV adapter at any Major RV delaer, Generator store or Camping World.. Might even find one at Wal-mart et-al.