I had the same idea for stepped rafters, but with more insulation in the middle and less of a "step" at the sides. The space above the cabinets would have a fore-and-aft cooling duct with outlet vents at appropriate places, sort of like BFL Johnny has is his ceiling. Intake vents would be at toe level, and efficient electronics/computer fans would gently lift air through a riser duct hidden somewhere in the walls. This would be kind of like having a concealed ceiling fan.
The business end of any air conditioning or swamp cooler could cool the air in the riser, so the floor-level air would be cooled as it was lifted to the overhead vents.
In cold weather, either the fans would be reversible, or there would be an alternate "faller" (as opposed to "riser"
) that would take in the warm air near the ceiling and discharge it at your feet. The core from an under-seat heater or other heater core could be in the "faller" so the fans would circulate warmed air when the engine is running.
The rest of the space further back above the cabinets could be used for wiring ducts for low-voltage, high-voltage, photovoltaic and antenna wiring.