This type of electrical problem could be caused by multiple individual issues - and perhaps a combination of them - and this may well be why you've spent so much on a mechanic thus far. Believe it or not, these weird little electrical issues can be enigmatic to a general mechanic too, and they'll be wary that they're getting in over their heads. You never know with wires!
Since LUV's have a reasonably simple electrical system, it's not uncommon to find that previous owners had taken shortcuts and, in a pinch, jumped wires to 'make do', and it's very possible that you or your mechanic simply haven't noticed the shortcut. Sometimes simple issues are not obvious, and short of replacing your wiring harness, the next best way to chase out electrical problems is to print the diagram, highlight the parts you care about(see below), and either go verify that the same situation is true on your truck, or do like I have done and bring your handy highlighted wiring diagram to someone who deals primarily with automotive electrical. It'll be like a cheat sheet to any decent mechanic, and he won't have to deal with the headache of all the initial troubleshooting due to inexperience with that particular model. Carbuerator mechanics are a good bet. Or an alternator rebuilder? You could try an auto sound guy as a last-ditch-effort, but they're the least educated of the lot.
The wiring diagram for the 72 is available in the reference materials section
http://www.luvtruck.com/Information.htm Look for "Series 1 (1972) Factory Wiring Diagram (ZIP)"
Print it big, don't be intimidated. It is laid out like a picture of your truck from above - headlights at one end of the diagram, and tail-lights at the other end. You'll see an obvious battery on there, and perhaps some other parts you recognize. It only looks complicated at first, but ignore that, and mark it one piece at a time.
Get out your highlighter and:
Find the "C light" you mentioned. It may be called "Charge light" or something similar.
Find the battery
Find the alternator
Find the ignition(key shape)
Highlight all, and get a ruler so you can follow the lines. They're all too close, I know.
Now connect the dots with the same highlighter. The wires you end up highlighting are going to be your mechanic's roadmap. Ask him to verify those particular connections and report any differences. I am willing to bet you'll find the smoking gun there.