Does your box have a 50 amp RV socket on it? That is 120/240 volts, and takes two hot leads, a neutral and ground.
If you have a 30 amp socket, you would take ONE hot lead, a neutral and ground to it.
Taking two hot leads to a 30 amp socket does indeed make it 240 volts, and will damage your 5th wheel.
Each hot lead needs it's own circuit breaker, so a 50 amp socket uses a dual breaker and a 30 amp socket uses a single breaker.