Decided the space under the floor was useless as soon as the Jeep is loaded up so added a third battery to the bank to make a 66AH battery bank. Runs the fridge for an estimated 38hrs at 32c, so longer in real life (with cooler nights), and this is without really drawing down the batteries too much. Just need to replace the leather gloves between each battery with some rubber sheet to stop one battery damaging another. I had to cut the carpet to get them to sit nice and flat. Real happy with it and have the fridge running now so I can check the voltage in 18 hours or so.
And why I didn't bother trying to use the space for anything else.
Cheers
Steve