If you look around you can still find the stuff from Oregon Scientific (and
others) that will take more than one remote temp sensor and in fact
different kinds of sensors.  My setup has two remotes, one for outside and
one for inside the 'fridge.  The one inside the 'fridge is a simple box.
The second one has a long wire for the temp sensor.  This is mounted on the
left side of the dash between the spedo/tach console and the A piller and
then the temp sensor strung along the iside of the front door jamb  and down
under the van and attached there.  The box itself has a readout on it so I
can see the outside temp, just above the ground, when driving.  The main
reciever is mounted on the interior wall of the hanging closet.  From there
I can see the temp inside the van, the temp outside and the temp in the
'fridge by selecting which channel I want to see displayed.  Had this setup
for at least six years, maybe a little longer, works great.