To put my two cents worth into the "auxiliary battery" discussion. Several
years ago I got tired of waking up in the morning to find my camper battery
dead and not being able to start the engine. I built an "Isolator" by
mounting two 50 Amp Stud Mount Diodes on a heat sink (Yeah! full wave is
overkill). Also on this same heat sink I mounted a "Ford" starter relay (the
kind that mounts on the firewall) in parallel with the isolator. I wired
this with wire that I had from an old set of "Jumper Cables" that the clamps
were bad but the wire was good heavy copper strand. The relay I wired to a
button under the dashboard. wah-la, instant jumper battery at the touch of a
button. This worked great for me, the only thing was that the auxiliary
battery would only charge to the level of the primary battery. Therefore if
the primary battery is bad don't expect to get a good charge into the
auxiliary. Anyhow, It sure beat begging the neighbors for a "Jump Start".

Neal