The WC54 are not really configured to haul a trailer. While they do have the pintle receiver, they did not come with the electrical connector for the trailer lights. Based on T/O there would have been plenty of other vehicles to haul the limited number of trailers a unit would have had available. I also have not found any photo evidence of trailers being pulled. From what I have read ambulance drivers were constantly busy. They would get cross attached to different units even when their parent units were moving forward to support new operations. With their constant movement, I guessing they didn't wouldn't have been used to move trailers as the trailers would have ended up in the wrong place.
That being said, I believe that there were occasions when the WC54 hauled trailers. Soldiers are good about adapting. If there was a need to haul a trailer and the only thing available was an ambulance, they certainly would have used it. In the Army, never say never.
On a side note, I have talked with Army nurses who got hauled around the ETO in the back of ambulances. They remember taking turns with their face pasted to the rear window to see where they are going.
PS I'll be adding an electrical connector to my WC54 when I redo the wiring harness next year