It sounds as though the Guest 2430/2401 are diode isolators. As such, they seem to be functioning as designed: they will always show about a 0.6V drop from input to output. (I think this is called the diode forward voltage drop, Vf). Not knowing the battery type (gel, AGM
or wet), it could take a very long time to recharge at 12.7-13.2V.
You might consider replacing them with switching-type isolators that show no voltage drop. Blue Sea has one, I think, for these levels of current
. Thinking about it, I am not sure why you need two isolators...
Alternatively, you could wire the charger and alternator directly to the house bank and reverse the isolators so that the house bank will always see the higher voltages. The start battery will then see the lower voltages but it should only require minimal charging
It sounds like the charger is an old ferro-resounat type. I found the investment in a new 3-stage charger well worth it in terms of battery life and ease/time of charging
- - Dave