Welcome to the forum Simon. Great choice to buy an Eriba. Very much away from the mainsteam caravan design. The Puck is very much the basic end of the range, so you can do quite a lot.
I considered fitting a leisure battery, but decided on a more temporary measure given that most of our time was spent on sites with power. I put a leisure battery in the car boot. I would attach an inverter that converted 12v DC to 220 AC and put this into the main caravan 220v supply socket. For powering the lights and other low powered items, it worked a treat. I know it is less efficient because you are converting the power twice in part (from DC to AC and back to DC) but for light use and just the odd night, we never had a problem. It is also worth noting that in the UK, wet leisure batteries fitted in a caravan should be in a separate compartment that is air tight sealed from the inside of the caravan, but vented to outside. That includes sealed lead acid batteries and gel batteries. Hydrogen Chloride is given off during charging, apparently even so called sealed batteries, which have pressure relief valves that can release gas during charging.
A lot of people do it though and just fit them under the bed/sofa. Good luck with whatever modifications you go for.