Not sure of Bill's setup, but I run a 350A solenoid on mine (Painless Wiring dual battery kit) and in normal mode, solenoid is on whenever the ignition is on and both batteries automagically charge when the solenoid is connecting them in parallel. Turn engine off, solenoid drops out and now I have two separate bateries. Main battery dies and I flip a switch to turn on the solenoid off the 2nd battery and I give myself a jump start:

dial battery install

And here's a photo of one of the connectors (middle-right), a +12V binding post (middle-left) and at the far left, you can just see where the ground cable run along the frame to terminate at one common bolt:



With the pair of disconnects, I can isolate the batteries from the rest of the truck, swap them for main/aux. use (I do this every few years to even the load on them), or tie in jumper cables to combine them in series for 24V. Makes for a very flexible and reliable system.