Nice rig...
We use power management devices at work for vehicles with a single battery that run on-board wireless modems, ToughBooks, and mobile printers. They have a sensor the prevents the voltage dropping below 12 Volts.
Always wanted to locate a programmable unit because cutting off your powered devices does not seem optimal because you don't want to be away running trails and come back to spoiled food and hot beer just because you went below a certain voltage.
Think you wouldn't want to run the deep cycle source completely flat, but there has to be a threshold you want to maintain to keep the battery from going bad from too many complete discharges. The battery we use is for big 12V stereo audio and the manufacturer documentation warned to stay above 9 volts, so keeping an eye on the voltage is a good thing. Some of the solar controllers have a digital display that lets you see the source input as well as battery voltage.
Just a few thoughts...
RDavisInVA