What you get with 50A service is 2 hot lines each capable of supplying 50 amps of current. The circuit breaker for these 2 legs is connected together so if 1 trips then they both trip.
With 30A service you only get 1 hot line capable of supplying 30 amps of current.
A 30 to 50 adapter just takes that 1 30 amps lines and supplies it to both of the input lines to the RV. You now have a total of only 30 amps being split up to a system that is setup for a 50 + 50 service.
This is actually the way most houses are wired (the 50 amp service that is), you have 2 phases of 110v going to the panel and sent individually to the house outlets. To get 220v you send both lines to the properly wired outlet.
If you were to connect a voltmeter to those 2 hot lines it would read 220v. Connecting the voltmeter between either of those lines and ground or neutral will read 110v.
2013 Coachman Mirada 29DSSE