Thanks for some really useful input.
I've just been around the truck again and there really is nowhere else to put the battery outside, other than in the truck cab or under the hood, neither of which are great solutions since I want something running in parallel with the existing camper battery.
I checked under the truck - very short bed and little space between chassis and body, so unfortunately can't hang a battery box there. Only 2 inches between front of camper and truck cab and C of G is already too far aft. Not enough space by wheel arches either.
Had Another hunt around inside though. This is the area I am playing with:
You can see the little battery tucked next to the water tank. The big space in front is where the portapotty goes. But if you look to the right there is a circular access hatch to the area where we keep our bucket and spare LPG cylinder. Having measured that area I reckon I can get two small batteries like my current one squeezed in next to each other in that space. Then fit the spare LPG bottle where the current battery is. Have to ditch the bucket though.
Steve.