Breakdown due to dead battery

You are correct. In series voltage increases, Ah stays the same as the lowest Ah batt, this is what causes the lowest Ah batt to sulfate. Parallel, voltage stays the same and Ah = the total of all batteries in the bank. They will balance as long as the amp draw isn't over the capacity of the lowest rated batt or the lowest rated batt will sulfate as well. My confusion is that I don't know if rigs have say 1 or 2 dedicated starter batteries and 4 house batteries for powering an inverter (fridge, microwave, etc) similar to what you'll find in a marine environment. Do they have dedicated starter and house batteries? If not, maybe they should? When the OP posed the question, I was thinking separate battery banks. And if they're hard wired between the generation source, the alternator(s), then you have a conduit for power drain.

 

A lot of things should still be taught in high school... like which bathroom you should use.

 

Replace entire battery group with identical necessary batteries that your OEM requires.

Test the Alternator output and so on.

Tractor trailer batteries are never onesies and twosies. Toss all of them and drop new ones in as a entire group.

 

After having worked for them, it doesnt surprise me that they would send you down the road with bad batteries.