............................................Bentley Manual and used a 40
amp relay ($10.00). The relay is switched with the same wire connections
used for the existing camping gear relay. The power side of the relay
can be taken from the camping relay power post but I ran a separate 6
guage wire from the main battery +ve post because I thought it might be
safer to not overload the existing wiring. For added safety I included a
40 amp fuse between this 6 guage wire and the 40 amp relay. This setup
can never drain your main battery from anything connected to your
auxiliary battery. Likewise only your main battery is connected when
starting the engine. This setup also ensures that the auxiliary battery
is only charged when your main battery is fully charged.............

This sounds like a great solution, but I do not understand the connections and
how they are isolated.  I basically have only three wires to contend with:

1) Wire from the main battery + post to the back for charging
2) Wire from the Aux battery + post to the back for charging
3) Wire from the alternator for charging the battteries

The relay is like a switch.  I understand if I put a switch/relay in line from
the main battery to the alternator that:
1) is closed when the key is on and 
2) opens when the key is off 
that I will not drain the main when I am parked, but how do I prevent from
blowing-up the battery that is over charging.  Do I wire the batteries
together, +post to +post and put the relay between and only run a wire from
the alternator to the main?  Wouldn't this just charge until the main was full
and none for the Aux?
                RELAY
GRND       ______o/_o___________________________
l          l                      l            l
l          l                      l            l
-          +    GRND__ -          +            I
I__________I           I__________I          __I__
I    AUX   I           I   MAIN   I         I ALT I
I          I           I          I         I     I