The way the stock heating sustem was designed in the later model buses,
air is pushed into the heater boxes directly from the main cooling fan
as well as an electric fan mounted on the 'roof' of the engine room.
>From there, it passes through two flexible tubes (one on each side)
and then into the gas heater under the belly.  Fresh cold air is added to the
warm air at this point, by means of another electric blower.  The gas heater
superheats this mixture of air and then distributes it to the defrosters,
heat ducts, and rear seat heat (I think the rear seat thing is non-westies
only) The point is that you can have your cake and eat it too.  When immobile,
the van can be heated via the gas heater alone.  When you are blowing down
the highway, with cold wind and puddle-splashes hitting the van at 60+ mph
you may need additional heating, provided by the heater boxes working in
conjunction with the gas heater. (vehicles without gas heaters get cold at
highway speed)

I think the confusion stems from the following-  That belly pan heater
_replaces_ the pipe everyone's talking about.  In buses without gas heaters on
their undersides, there is simply a thin walled pipe with insulation wrapped
around it to carry the heat all the way up to the front.  Later models have
plastic pipes, and are not subject to rust there, but 68-74ish buses have this
problem. Speaking for bubble-windowed buses, here...  ^^

Harris