I have installed this product on a couple of emergency vehicles and a couple of mine over the years. http://www.prioritystart.com/ It monitors your battery's voltage and automatically disconnects the battery when it starts draining below a pre-set voltage. When you turn your key on, to start, it re-connects with enough power left to ( hopefully) start your vehicle. It has came in handy on more that one occasion.
I only had one fail, of course it was on one of my cars ! Manufacture replaced it..
Future mod list; auxiliary battery, also; I have used "Marine" switches with " One-Two-Both-Off" selections..on a couple of my other vehicles.