I have decided to try the following to connect an auxiliary battery in
my 87 Westy:

Install an Optima battery in the aux box under the driver's seat.
Connect the negative post of the aux battery to ground.
Connect an 8 gauge wire to the post of the starter battery.
Connect the other end of the 8 gauge wire to a switch to be mounted
behind the driver's seat.
Connect the switch to the positive post of the auxiliary battery.
Connect the refrigerator wire to the auxiliary battery positive post.
My intention is to leave the switch open while the engine is running
and switch off when the engine is off so that the refrigerator runs
off the auxiliary battery.

I don't regard this to be an ideal solution but I'm planning on
leaving on a 2-week trip in a few days and would like to have an
auxiliary battery. (I'm refurbishing the propane tank and won't be
finished until I return.) I've looked for a 75 amp relay to no avail.
I've looked at isolators but I don't like the idea of installing it in
the engine compartment. (I bought one from JC Whitney and it requires
connecting the alternator leads to the isolator.) I've considered
using the existing relay cube in the aux battery compartment but all
of its jacks are used. Any comments are welcome as I am not an expert
with electrical systems.