Engine off, If you have 24v at the alternator 28v bolt to ground, that is coming from the batteries. You can get 24v from that bolt to apply to the exc terminal on the reg with a small piece of wire. But since the alt is going over battery resting voltage I suspect it is probably getting what it needs at that exc terminal on the reg, thru the normal excitation circuit.
what amp rating is this alt?
The problem with 4 batts is that a good batt in parallel with a bad one can mask the bad one, so this could still be a batt issue. AGMs when depleted also want nearly twice as much charge current as a similar AH rated wet cell. You cannot leave it like that, but you can certainly disconnect two for troubleshooting.
this is as simple as loosening only the battery terminals on the forward(toward front of truck) pair or rear pair of batts, leave all other interconnected wiring tight and loosen and lift the terminals up off the posts and slide something under them to keep them isolated from the posts. This will drop you to a pair of batts in series(one inner, one outer).
do you have a batt load tester? I described one above with headlight bulbs, but a pile type loadtester is even better. Do a 100A load test on the batts, ground to 12(outermost pair of batteries) and 12 to 24(innermost pair of batteries) note the drop with 100A applied. Do the same test at the alt ground to 12v bolt and again across the 12v bolt to 24v bolt. The voltage drop should be the same As at the batteries
A voltage test alone is not a complete test. It is not valid untill tested UNDER LOAD… a bad connection will pass full voltage throughout a circuit untill you try to pull current thru it.
if one of the circuits between battery and alt is breaking down with current flow, the alt will be unable to sense the batteries properly. This is not just a regulated alternator it is a battery balancer. it MUSE SEE 12V from the middle of the 24V series battery under load/with current flowing. if it does not see this it will not regulate properly.
I have seen this on my own alt while experimenting and learning how these things operate, and it delivered voltages similar to the ones you described.
the only way to truly test that circuit is to do a low amperage 12 and 24v battery load test AT the alternator terminals...