Diesel charging issue. Let's keep the OBD stuff out of the way for the guy...

As said, make sure you have good battery connections, alt connections and check the feed wire on the alt. If you can't find anything on the visual, time to get a voltmeter and check the battery(s) and the alt output.
Thos old trucks tend to have cruddy connections after this much time and miles, so a good look/clean can often fix stuff.