When a 30-50 amp Adapter is used with a 50 amp Service, the single 30 amp leg is connected to both of the 50 amp legs by the internal jumper in the plug.
50 amp service provides 50 amps on each of the legs, 100amp total. 30 amp service is a single leg at 30amps.
Complete discussion of RV electrical is available here:
FTLS - Electrical Distribution