I really don't think that's a return. In our coach a BC 4010RD, we have a vent in the stairs but it is just open to the area behind the basement wall. I figured it was to allow coach heat into that area for the plumbing, since the "heated underbelly" duct goes from the furnace down below into the lower area under the floor where tank valves are and such.
We have a heater return above the pantry/cabinetry in the kitchen.
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