I would side with Rich and others in using a solenoid switching system. Christo has a wiring diagram on his site that can be used as is, or modified for your particular application.
The
schematic is here.
I have used a variation of this set up for some time, with winch, radios, etc. and it works quite well. Here are the advantages
as I see it...
1. Less expensive;
2. Installs in a smaller space;
3. No voltage drop;
4. When discharging, batteries are isolated
only when engine is
not running...
This last point is significant (to me) because when you have a high current demand with the engine running (winch, lights, radio [in transmit], etc.), you are better served with a pair of batteries in parallel, than just a single one - or the other - depending on how you have wired them.
The major "disadvantage" of potential inrush of current from a charged battery to discharged battery is mostly hype, and can be avoided if you wire the system so the batteries do not "see each other" until
after the alternator is actually putting out a charging voltage.
Cheers, R -