How much solar and how much battery power per bank - it could make a difference.
There are guys doing creative things with hook ups. Some of them end up with a requirement for an instruction manual to turn batteries
on and off especially if switches are wired in series.
For me nothing is as simple as paralleling relays. I don't see any reason not to have multiple paralleling relays and when charge sources are available the batteries all tie together.
The problem is, and I am guessing here, is that you probably want to deplete a bank, switch to a full bank and then charge the depleted one in isolation. I am guessing also two larger house banks and one start battery.
There was a recent thread on this but I can't locate it readily. It had a wiring diagram in it.
I could envision a pretty automated system for this using solenoids with manual overriding. Doing it manually is easy. Doing it automatically is easy. Doing it automatically with large range of manual control is a little more complex.
Blue Seas has a fair amount of equipment
that could do all this if put together properly...