In using one 500A service you would have to distribute to seperate trailers. In the method you "prescribe", it seems that you could provide the same type of sidtribution, just from 3 transformers. I can't see any benifit of having one 500A over 3 seperate 200A services. Whos drawing does this requirement show on (architectual or electrical), it might lead to whether it was a stab in the dark or a calculated/particular requirement.
