Does such an object exist that opens a circuit at low volts?

never mind.. what i was thinking won't work

"Does such an object exist??"....Yes, as a low voltage shutoff feature in an affordable charge controller. Can easily be wired and used to perform the function that you so desire:

http://www.specialtyconcepts.com/special_home.html

Yes, an EMS from Progressive Industries.

There are MANY low voltage cutouts.. Some reset automatically, Some manually,, Some are for low current DC Some HIGH current DC

I have one that is for 50 amp times two legs AC (We call it a Surge Guard).

There are dozens.... I have not looked for them in recent editions of "The Handbook" but I recall seeing Make your own (for DC) some time ago in one of the Ham Radio publications.. Alas too long ago to give a clue.

that morningstar system is nice, going to cost a lot more than a DIY aurdino system

what ever is involved will have to go between the batteries and the parasitic loads
unless you want really heavy duty relay contacts, this means placing it between the DC fuse center and the parasitic loads
really if the batteries cannot carry the parasitic loads for a few days and the solar cannot recover, you have bad batteries and two little solar

yes i understand how bad the weather has been in Texas and this is not your normal situation
why not just disconnect the loads and leave the solar floating the batteries, obviously your talking detectors and alarms, not the fridge and appliances,
disconnected manually automatically or by dead battery..they will not be working

alarms are for when you are there, they nothing but make useless noise when there is nobody there to take action

Morningstar has a Relay Driver to control four relays to act as you like.

Relay Driver

Yes, it's called an undervoltage relay.