I know battery issues are done to death, but I was wondering about
something...

I want a little device that will detect voltage from the auxiliary battery,
and cutoff the whole aux circuit panel below some threshold (11.2 volts, or
whatever).

that way, if something gets left on, it can't trash the aux battery too bad.

Any simple devices that I could combine with a relay?  Obviously, I want a
setup with very low operating current, since this will be on all the time.
Ideally it would also have an override switch, but I wouldn't mind adding
that feature manually if I had to.

Thanks for any tips!