I have an '86 Vanagon Westy 1/2 camper that has an aux battery. I don't know
if it was fitted from the factory this way or if the previous owner did
this.

Assuming it is a factory installation, how does the battery cut-off relay
work? Is this one of those battery isolators? I don't know how those work
either.

Any other advice about the aux battery?

Thank you,

Jon Glommen
Product Engineering
NOVELLUS Metals Group
408.570.2814