Winnebago provides online wiring diagrams:
for a 2006 Itasca Horizon 36RD, the diagram set is here:
2006 Vectra/Horizon S36RD Wiring Diagram Book
Your coach may not be a 36RD; use the correct set from the first link above.
The specific drawing in that set for the 120V, including the EMS, is here:
You see there that the dishwasher receptacle is connected to the "LINE2" input for the EMS. As Chris Beierl has said, that breaker must be on in order for the EMS to show "50A" in the display.
[Addendum: I may be incorrect, but it appears as if the LINE1 input comes from the first 15A breaker on the inverter subpanel, which feeds the central vacuum. Just guessing, though. You could test by turning off that breaker and seeing if the EMS display shows "30A" on loss of the LINE1 input.]