.............. The installation of the propex puts both the intake and
exhaust pipes/hoses within about a foot of each other under the van,........
Not correct, at least in the implications.  The Propex exhaust is outside
and if properly installed to the side and away from the van, BUT the intake
for the HEATED AIR is from inside of the van through the grill/holes in the
case of the heater itself.  This seperates the two sources and provides one
of the nice things about the Propex, the recirulation of the interior heated
air to provide more efficient heating.