In article JS writes:
How about this, diagram below.
Each battery should have its own fuse.
Switches are drawn as -o\-
Vertical line is the Master bus.
Optional PV charge controller charges the active battery, whether avionics are on or off.
I'd suggest locking switches like Alcoswitch MTL106D available from Mouser for $12.60.
Jim

Bat 1 -----o\-----|
Bat 2 -----o\-----|
Bat 3 -----o\-----|
|-----o\----- Avionics
PV Charger -------|

I would seriously recommend against this.

If you only turn on the avionic load switch, with no batteries in
the circuit, the avionics will see the full open circuit output of
the solar panel.

Many "12 volt" panels deliver in the vicinity of 21.6 volts open
circuit -- even if not that high, they can easily be enough to damage
electronics.

Charge should be applied through a charge controller, so the panel
doesn't overcharge and cook the battery, and should be applied in
such a way that it cannot be applied directly to the load alone.

Alan