ST/PSR122
A successor to the old diode based splitting systems which are known for their high voltage drops. The new intelligent ProSplitR selects a battery bank and isolates the other banks to prevent their voltage misleading the alternators regulator. This allows the regulator to focus on the correct battery and maximise the alternators potential. Then at a specific level the other battery banks are charged and finally all charged together.
Does exactly what you want it to do but a word of warning, it does not last forever. The relay inside will wear out. My first one lasted over 8 years of heavy use so not unreasonable. Sterling staff are very helpful.