Batteries for the habitation side of your motorhome differ greatly from an ordinary starter battery found under the bonnet. They are designed to be discharged more without harming the structure of the battery and also for faster charging. Not all leisure batteries are the same. They vary by type, wet cell, advanced flooded cell both usually sealed, gel type, and lithium. Also some uncommon types are not 12 volt and are wired 'in series' to make up the 12v circuit. But do not concern yourself with the lingo, basically and for most applications we will look at wet type batteries. They also come in 3 catagories, A/B/C with A being the best. Capacity is important and the more the better. More than one battery can be fitted to increase capacity.
If your van has room to fit additional batteries then consider doing so, especially if fitting a solar panel setup. It makes the 'free' power generated storable and usable later. Also consider a B2B smart charger (battery to battery). This utilises the more modern vehicle alternator technology and tricks it into supplying a high current to charge the leisure batteries when the engine is running. When fitting batteries they should only be fitted together, that is.... fitted at the same time, new, from the same batch, fully charged and linked together properly. We can offer full and impartial advice on your requirements if required. Finally, if you want to have 240 volt AC mains power then an inverter is required. Click HERE for further details and what is required.