Re isolation of spare battery in camper......
All you need is a SWITCHABLE (hence the relay)  wire going from the regular
battery (+) to the the spare (+), and a GOOD ground for the spare battery.
The relay simply detects when your ignition is on (why it's (+) goes to the
ignition terminal on your coil), and then allows the heavier (+) from spare
to head on down to the real battery so it will charge in parallel with the
real battery charging.  Those with GOOD memories and a desire to finish
that movie on the portable VCR when voltage drops too low in spare when out
in the boonies can install a $1.29 switch in a 12gauge wire running right
between the two battery positives.  The relay is more expensive, but any
relay that will pass the charging current will work.
Since it's ALWAYS ON when ignition is on, it's too late to know relay isn't
working when your spare is run down- hence the good idea of some sensor
across spare to tell you it's actually charging- You can use the simple
colored diode thing that plugs into an aux. socket (from radio shack) and
watch it when car is on and off, or use an aux. plug wired into a simple
digital voltmeter.  The latter gives better info, such as seeing charging
voltage when battery is connected, vs voltage to real battery when you yank
the wire to coil, showing if your regulator is doing its job or if
alternator can handle it....  When my spare poops down at the campground
and I don't want to run the car, I get out my handy jumper wire (a 12gauge
wire with big alligator clips at each end and connect + to +, and wish I
had put in the switch next to ignition keyhole to manually isolate or
connect the spare!!!  MB in NJ