Where to install a VSR

Tazbaz
Whilst an Ignition switched relay is cheaper and could work satisfactorily on some vehicles the problem with using such a simple system in the Toyota (and many other) vehicles is that you are effectively starting the vehicle from BOTH batteries.

Whilst this may at first appear to be beneficial the down side is that you may (will) never realise that the main starting battery has lost sufficient capacity to carry out its primary role, that of starting the engine, until the auxiliary battery has been used for its main role of powering the camp equipment and is then in a low state of charge which is also not capable of starting your engine. You have then negated any benefit of having a dual battery system.

The only way to run the cheaper (non VSR) relays with any certainty of being able to detect a starting battery pending failure is to have the relay triggered from some source that is totally independent of the ignition circuit. This MAY be possible from the accessories (but not always), or in the case of the earlier Isuzu products by accessing the Air Conditioner circuit which was only active after the alternator had commenced operation. This does not apply to the current Isuzu range of light vehicles, so they also now require the VSR for satisfactory battery isolation.

Regards
Athol

Athol is dead right,
I have come across that exact problem . With the ignition switched relay you can have either battery die , and need to test them independently to work out what's going on.
I use vsr's and have a push button switch ( as a lot people would, but some don't bother hooking one up) so can manually hook both batteries up when needed.
Also I find it's not a bad thing that when my auxiliary battery is up to the vsr voltage the start battery also gets charged from my solar.

Woops , that's all off the op's question.( I'm guilty of an annoying trait that seems to happen to often here)
Where can it be mounted? Someone with the same vehicle may be able to help.
Cheers and good luck, hope someone answers the question!

"Tazbaz
Whilst an Ignition switched relay is cheaper and could work satisfactorily on some vehicles the problem with using such a simple system in the Toyota (and many other) vehicles is that you are effectively starting the vehicle from BOTH batteries."

Not necessarily. The smart cookies power the relay from the .circuit. That circuit gets switched off when the key is in the start position

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Peter, you said....
"smart cookies power the relay from the .circuit".... You seem to have left some text out.
Did you mean from the keyed 'Accessory' circuit?

Hi Alan, yes it looks like a bit of the text whilst I was struggling wit a few attempt to send the message. I here with correct the error and add a bit of extra.

Not necessarily. The smart cookies power the relay from the accessories circuit. The accessory circuit gets switched off when the key is in the start position. That means that the deep cycle battery will not be connected to the starting battery whilst the starter motor is working.