Usually there are two separate duplex receptacles (one washer, one dryer) in the closet up and to the right of the water valves/drain box. At least in my Landmark.
What trailer do you have? Others with similar trailer may post a pic of the washer dryer prep in their unit.
Brian
EDIT: Just dawned on me that units plumbed for the combo washer/dryer may only have one receptacle. I have the separate washer/dryer stackable.