Don't know why VW lost the rear heater on the campers but:

The rear heater is mounted behind the passenger seat (you know this) and does
have a control in the dashboard for fan and temp.

What is bad is that this control is electrical, not mechanical and the valve
would be very expensive. Also, many, most of the heater hoses next to the engine
will have to be changes too. Installing the coolant lines back to the heater
requires the right suspension to be removed too. (never done it, thats just what
the book says)

 Your best bet is to find a junkyard to get the parts but I don't think this
will be very likely.

My advise would be to get a Rear heater from a vanagon and make up the
conversion yourself. This way the heater can be mounted way back and be more
effective.