You must be careful about two independent 120 volt circuits. They must be on opposite side of the 120/240 power source or you have the potential of frying the neutral.
Wire to the electrical panel should be a minimum of #10 for proper operation. Breaker should be a double pole 30 amp. This should handle everything thin in a 50 amp rig.
Depending on how your location is constructed and location of the electrical panel to the rig, best would be to wire it complete for a 50 amp 120/240 circuit with 50 amp breaker and proper RV 50 amp receptacle.