1998 kenworth voltage meter dropping

Personally I would start with checking the charging circuit via voltage drop checks. Thing is you need the system under a decent load so flip on every accessory you can with the truck running. Put a digital multimeter lead on the alternator positive stud and the other on each battery positive post. Any voltage you see indicates a voltage drop (loss). Basically a poor connection. Do the same on the ground side - alternator housing (or ground stud if it has one) to each battery negative. Same story, any voltage showing on the meter is a loss.

If you read a voltage during the above tests you can work back from alternator to batteries until the meter starts reading no (or very low) voltage, that would indicate the issue lies between your last 2 test points.

You can test for voltage drops on any circuit that way, even cab or engine grounds.

Obviously use care and attention when working with test leads around the fan, belts, moving parts etc. Don't want to get caught in those.