Thats kind of surprising, it is usually the 12v batts that are toast as the A0 has a 12v vampire and because of the lighting loads, the 12v side has trouble fully recharging with the undersized alternator.
4 wet cell 6T batts is way too much for the 100A dual volt alt. Anything AGM even more so... It is basically two 50A alternators trying to feed 240AH at 12&24 respectively. If discharged those batts are looking for 60A from the alt(25% of AH capacity in amps) All by themselves, so instant long term alternator overload Just from the batteries, not even starting to count the truck loads on top of that(lighting is 22-26A).
you have two that could feed 1000A, install one of those in an inner/closest to frame, 12-24v location and the other one in an outer/furthest from frame ground-12 position and wire them in. A pair of 6T wet cell batts at 120AH are still a little large when you include truck loads, but are a much friendlier match for that horrendously expensive alternator. they are also plenty of energy for the truck…
If these are hawker AGMs, then even a pair of those is too much for the alt. AGMs are looking for 45% of the AH rating during charge, so a pair of AGMs is nearly as much load as 4 wet cells. The alt and battery combination on the LMTV is one of the largest engineering mistakes I have ever come across. Sadly they occasionally happen in the land of milspec… as an example my 2000 Tacoma has a group 24 battery with about 70AH capacity. That batt is looking for about 18A out of the alt to charge. It has about 15A of truck load when everything is turned on. It has an 80A alternator, so less than 50% load worst case. Most autos are setup this way. They got it way wrong with the LMTV. They added the LBCD to disconnect the batts to save the alt on the A1, and finally fielded the 260A alt on the A1R, but because they introduced the AGM batts about that time it was an absolute necessity anyway.
If you are getting new batts, a pair of group 31s is an excellent match for the alt. A pair of these is also what Cat specced for the power equipment that uses these engines, so it is plenty of power to start and operate the truck. Your alt and your wallet will thank you.
I run a pair of 31s in mine...