Yes the Traxide unit is simply brilliant, can't rave about it enough...
I use a circuit something between the first and second diagram, but I use two marine switches which allow me to manually connect all three batteries should the need arise. Have an 80 litre fridge/freezer which can really chew some power. I was just wondering if anyone had a valid suggestion on whether to put it in the back with the auxillary batteries - like I say the Traxide rep (who is extremely helpful) said it would be fine under the bonnet so thats probably where it will live. Then I will place a 100amp circuit breaker at each end of a battery cable ran up to the back and treat the two auxillaries as a single battery (even while typing I am indecisive...lol).
I am into electrics/electronics - fell into IT after doing a electronics based course so I love playing around with the wiring.
Been out measuring tonight so I can weld a tray up for the back of the car - I'll post pics once done.

Thanks for your help.
Rokkitt