Bruce,

I own a 78 camper and have the same problem as you.  I completely removed the
refrigerator in attempt to find out what the problem was.  It turned out to
be the fuse in the transformer located under the bench seat. There is also an
in-line fuse that runs from the battery that should be checked.  It helped me
to draw a diagram of the electric flow.  When working properly, that element
will get very hot and send the gas through the cooling system.  It gets
pretty cold, but not quite as cold as you would like it.  Mine will run off
electric power but not while driving.  I haven't yet taken the time to
investigate this problem yet.  

I also have an airconditioning installed in mine.  It works fine and I have
been glad on more than one occasion that I have had it when I lived in GA.
 Remember, it will be hard for you to find freon.  My clutch went on the
compressor I had but I was able to find a used one.  You will also need to
make sure you get the right belt to run it.  It takes a special VW belt.  I
am sorry but I can't remember the part number.  Have you had any problems
with the compressor resting on the engine support beam?

Good luck, I hope this was of some help

Todd