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 -------|