Another issue not talked much about that may exaggerate the effects of different types of combined batteries is - "are any of them that are connected in parallel connected in a balanced parallel configuration".
Even if matched and/or new identical batteries are connected in parallel wrong, then they can over time age differently such that one battery or set of batteries eventually becomes weaker than the other battery or set of batteries due to unequal sharing of load and charging currents. This applies to 6 volt batteries where dual sets of them wind up in parallel with other 6 volt dual sets of them and it applies to 12 volt batteries where any quantity of them wind up in parallel with each other.
