The change from 30A (single phase) to 50A (two phase) could be done, but what is the motivation for doing it? Are you tripping breakers now? Do you want to add a second A/C? As Bill Adams detailed a 50A system is really dual wiring systems. One is 120V/50A via one phase of the source, and the other is 120V/50A from another phase of the source. (240V is available if you connect from phase A to phase B, but this is rarely used in RVs).
If you simple need to add a 2nd A/C you could wire the shore cord, transfer switch and new breaker box to the shore power, then use a 2phase/30A breaker for the main breaker. One phase would go the existing wiring (which is sized for 30A) with no changes, and the other phase could go to the 2nd A/C. This would not give you the total 100A of capability, but would allow two A/C's and a total of 60A of service. The only new wiring that would be needed would be from the breaker to the 2nd A/C. This might be a lower cost option than a complete re-wire of the coach.
