I assume you want propane heat for trips other than Alaska in the
summer.  Most of Alaska/Yukon is plenty warm enough for camping in
the summer without a heater, especially in late June/early July when
there is hardly any "nighttime".

How many days a year do you need propane heat?  Remember, while
driving water cooled vanagons you have great engine heat (compared to
air cooled busses).

New undervan propane tanks are available from GoWesty.com,  You may
find a used one from one of the used parts vendors that frequent this
list.  Install an extra T fitting so you can use disposable propane
cylinders as a backup and carry a couple extra  with you.

Many people like Propex heaters, expensive but nice.
http://www.thesamba.com/vw/forum/archive/index.php/o-t--t-145318--.html

Some people have even mounted them under the rear seat.  Cool.  I mean hot!
http://volksweb.relitech.com/party/1996/propex1.jpg

There are also generic propane RV furnaces such as suburban from big
RV suppliers
http://www.pplmotorhomes.com/parts/rv-furnaces-suburban-atwood-parts-1.htm

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If all this seems excessive/expensive, just get a  (relatively) cheap
heater buddy and crack a window a bit.

http://www.mrheater.com/productdetail.asp?id=678
http://www.mrheater.com/productdetail.asp?id=784

And it you are doing true winter camping, good insulation will keep
you warmer longer and save propane/electricity.  GoWesty sells this
kit, you could probably create something similar if you have the time:

http://tinyurl.com/angku