I used 2" of Foamular on top of the bus's metal floor:
Put plywood on top of that and you're golden. You don't need to frame the floor first. That rigid foam can support LOTS of weight.
I screwed the plywood down, but if I were to do it again I would have used all adhesives (PL Premium). The screws transfer some cold into the bus in the winter. It isn't enough to feel, but you can see ice crystals form on their heads then when the bus warms up the ice crystals melt and make little wet spots.