Actually made a start on this today, decided the best place for the battery that was accessible all the time was on the outside of the trailer.
This meant removing the 9kg gas bottle holder and 1 of the jerry can holders, 9kg was too big anyway so going with a 4.5kg holder.
Gas bottle holder removed.
https://www.aulro.com/afvb/images/im...015/09/280.jpg
Jerry can holder removed intact so can re-use if i want.
https://www.aulro.com/afvb/images/im...015/09/281.jpg
Battery and gas bottle in place, its 3.7kg bottle so slightly taller and narrower than a 4.5kg bottle.
https://www.aulro.com/afvb/images/im...015/09/282.jpg
I will be making an enclosed box that will go from guard to side where gas bottle is sitting, this will give me room for 2 x 100ah batteries if i find the 145ah is not enough.