Changing from 30A to 50A is not as big of a problem as many people claim. Provided you have room behind or very near the existing panel for another small breaker box. Cost is also minimal except a 50A shore power cord is not cheap. Excluding the power cord, cost is under $20 for the breaker box plus cost of the actual breakers. I converted our '04 Cruiser several yrs ago.
GE makes a small 6 or 8 slot panel, good for 200A. Two slots are taken by the dual 50A main. One slot with a single 30A breaker feeds the original panel which is now a sub-panel. Then it is a matter of deciding which circuits to move into the other remaining slots. Generally there is enough slack wire in each of the circuits, that if the new main can be located near the existing panel, it is a simple matter to unscrew them from the panel and move them to the new panel. As I recall, I moved three circuits, one on the same phase as the 30A feed to the old panel and two onto the opposite phase. One plus with this change, I was able to move the convertor onto it's own breaker in the old panel rather than having it share a breaker with another circuit.
I do agree that running a separate 20A circuit is cheaper (because of the cost of the 50A power cord) and may be easier. But 20A + 30A is only 50A, which is only 1/2 the total power of a true 50A setup. So it depends on your real needs and talents.
Edited by: Dayle1