I was told that a regular battery isolator will not work in a Vanagon by
someone who was told by someone who . . .

I did not believe this as my understanding is that a battery isolator is
just a pair of diodes on a big heatsink.

But, then I looked at the JC Whitney website catalog - they list several
isolators, many of which are described as not suitable for all VW vans!

Why is this so, and what do I need?