Tully, there is a lot of good information available on different trailer wiring issues on the net. Here is a good source of help. You can get a 6 way to 7 way adaptor in male or female but I'm not sure what you will loose without looking at the wiring diagram.
http://www.etrailer.com/faq_wiring.aspx
If your bus is going to be the primary tow vehicle for the trailer, I would insist that the trailer manufacturer supply the trailer with the proper hookup.