Semi trucks have this issue of uneven charging in their battery banks. Usually the battery farthest away from the alternator is undercharged while the battery closest is overcharged.
Gotcha. I wondered if that was what you meant.
Am mot so sure that is overcharging versus a single battery out of phase with the charge profile. For overcharging, I'd expect a failure in the alternator's ability to limit voltage to 14.8 and apply higher voltages. Although, would not surprise me if some OTR drivers defeated the voltage regulation to get a faster charge which both overcharges and cooks; especially if it is a company truck where the driver does not directly pay for the repair.
Given the description, it matches an imbalanced setup which allows out of phase situations where the battery closest to the alternator is cooking off its fluid as it wants a lower charge rate, but a battery further down the line still wants a higher charge rate. Also, the comparison is not equal as in the OTR setup, we are looking at much larger capacity batteries of equal size. In the DVR setup, we are looking at a setup of the OE battery plus one a small fraction that size. Basically, it is all in the engineering.
For the DVR setup, chances of cooking are there, but inherently low. Distance from the alternator (not placing it in the engine bay, but close to wherever the DVR and battery will fit), use of an isolator, and use of nearby wiring (or a dedicated 14 - 16 gauge line) will all reduce the amount of current going into that circuit. Having a permanent load on the DVR battery will also help limit the ability to cook it as well seeing as the SOC will likely lag the alternator's output (specifically, want bulk when the alternator is in absorption, or want absorption when the alternator is in float); actually, this should help the battery life.
Do agree that a poorly engineered setup could lead to cooking the dedicated battery, although in this setup am not seeing the potential as the alternator will not see much more than the the load of the DVR system.