Westernstar 4900 voltage dropping.

Alternators also have an amperage rating. The diodes convert ac current to dc current which is what the system and batteries use. Alternators get weak because one or more diodes in the alternator go bad so not all of the current is converted. Either buy a tester or have the alternator tested. Preferably one of the newer type conductance testers. Remove and clean all your grounds and coat with silicone dielectric grease.

 

You were checking DC volts, the 14.2 is volts. Amps need to be checked. A multimeter won't go anywhere near the amperage that an alternator puts out. Need a special tester or have a shop test it for you. Bad diodes can also cause a drain when the vehicle is off. There should be a tag on the alternator that tells the output, somewhere between 160 and 320 amps.
My bet is that the alternator amperage output is weak and not fully charging the batteries when the truck is running. First step is to test the alternator.