What I would probably look for is a hole in one of your heat 
exchangers.  I had this problem with my '70 bus when I first got it.  
It's not possible to see the hole do to it being inside the exchanger, 
but when I replaced it, the problem went right away.

If you disconnect the aux heater, does the smoke go away(assuming the 
other heaters are still connected =)).  If so, then it's likely to be 
one of the heat exchangers.

my .02

Jon