Neither coil voltage offering is ideal for Vanagon use. There are
various reasons to choose one over the other but it all comes down to
trade-offs. The actual voltage seen by the relay coil will depend on how
you wire the control circuit for the relay coil.

I agree that typical use of the relay in a Vanagon could be expected to
subject the 12v relay coil to in excess of the 110% spec. Probably 115%.
How often this will occur would vary by application. This has not
resulted in any known failures of the 8 or so I have installed in the
last 12 years.

Selection of the 15 volt coil version complicates the use of the relay
to connect the aux battery to the main battery for starting assist
should the main battery be low. Some people don't care about such a
feature but I want it available

My typical installation of the 120-901 is in Westys, where I use the
output of the stock fridge relay to control the Stancor relay mounted in
the driver's side battery box. The voltage at the fridge relay when the
fridge is running on 12 volts can be down at the 11 volt range depending
on other loads.

In installations where there is no fridge relay I use the output of the
X circuit relay from the dash wiring to trigger the Stancor relay.

Incidentally, SurePower uses that relay with the 12v coil in their
1314/1315 Battery Separator products.

Mark