I will assume the pan is concave or bowl shaped and the drain hole is in the center????
I would build a wolmanized wood foundation that will accomodate the pan and that wood frame would be bolted with stainless carriage bolts to the frame or undercarriage of the bus. You might even consider applying mortar to the wood frame that would be congruent to the bowl of the pan so that its equally supported.
I would build the pan with 2x6's and treated 3/4 ply.
Should be an easy task.
Position the drain so that its not near the tailpipe or any other integrat part of the undercarriage.