Jim,
I really do not know where the transfer box is in a 2003 given my motorhome is a 2002. In the 2002 Marquis, the transfer switch was located on the ceiling of the storage bay near the inverter charger. If your inverter charger is located there I would suspect that the transfer switch is also. It was housed in roughly a 10 by 10 by 4 inch Grey electrical box. You may also be able to follow the large AC output cable from the generator which should terminate at the transfer switch. Hope this helps.
later Ed