Hi Russell, Your best friend in this situation, is a trouble shooting light, which are cheap, & can be found at any auto supply place, or even reject & cheap shops.
You simply, hook the earth clip to the chassis somewhere, & then probe at various points from the battery +ve terminal, until it all of a sudden, the voltage isn't there.
There is a "master fuse link", that doesn't even look like a fuse, but is usually mounted close to the battery itself.
That will show up the open circuit, very quickly.
Let us know how you go, & what you find.
Cheers Banjo