[rpmerf]

That will do it! Should be less than half an amp, and in a vehicle that old, I would expect closer to 0 when the truck is off. I might be looking at another alternator also. 13.0 seems low. I would prefer to see closer to 13.5-14.5v while running.

Some chargers won't charge it if the voltage is too low. Hook it up to the jumper to get the voltage up, then hook it up to the charger.