Switching from 12V to 6V is almost never a semi-remotely-decent idea,
But this may be the exception?
The RC club house uses lights maybe 2 hours a week on Thursday night?
That could make it an OK idea.
It would require a rather expensive switch, and/or a bit of know-how, but it could work.
The batteries would be at approximately the same state of charge all the time. At the same voltage while charging, and the same drain while being used.
I did a 24V PV-powered coal-fired hydronic radiant heating system (for winter), that 'switched' to 12V use (for summer).
It's about the same idea.
If interested, I could dig out my paper sketches of the wiring and draw it in an 'MS Paint' jpg sketch.