Battery and electrics

Good morning together. Since the WE are proud owners of a Knaus Südwind BJ ’91. Great venture in which everything is actually inside. My question: I would like to expand existing halogen spots, and build up a reasonable battery. So far a small gel battery (motorad size) is set in a upper cabinet – loose to take out. But this retreat is empty. Where does the battery belong? Seat box/bed box? Which battery is meaningful? Would you like to watch Autark TV etc. as said am beginner of the whole matter and would be very happy about tips.